Picture a battered old Ford Escort. Dents in the hood, scratches in the paintwork, mud splattered on the undercarriage: all the evidence of too many frantic getaways and chaotic car chases. Crumbs on the dashboard, newspapers stuffed in the footwells, a permanent cup of coffee wedged next to the driver's seat. A frayed blanket pinned to the back window, a handy shaving kit hanging from the rear view mirror, and an infinite supply of parking tickets spilling from the glove box. All the comforts of a well-loved mobile home. My home.

Also my private office, and open shop floor.

I am Jack Hansard, the nation's leading salesman of Occult Goods. I sell from the boot of my car and live in the rest of it. No good being tied down in this business. Always best to have a handy method of escape – that is, business relocation.

I've had to 'relocate' a few too many times recently. In the space of two weeks I've fled two cities – London and Worcester – and that business with the Sandman was the last in a long line of close calls. Time to get away from the cities, it seemed to me. Get a breath of fresh air, so to speak. The business travels with me, after all.

I decided to take an unhurried, meandering route North to the Lake District, avoiding main roads and motorways as I habitually do. At least, that was my end destination. As it was, I got lost in the twisting back-roads of Shropshire – the sleepy shire, I've always thought – and after having wasted half a day driving in circles, it was time for a pit-stop.

I chose to pause in Ironbridge, a picturesque little location in the heart of a tree-laden valley, though the trees were still bare from the frost of winter. Tourist spot, based on the number of telling brown road signs I'd passed. Particularly proud of their bridge, it seems. I hear it's made of iron.

World Heritage Site too, according to that sign that welcomed me as I drove in. Lots of museums on industry. Pretty big on iron all round, it seems. Oh look, there's a Museum of Iron. Inspired.

Well, I did fancy a holiday (a working holiday, at least). It's not quite the seaside, but it'll do.

I pulled into the disabled bay outside a riverside pub, and pulled out my blue badge. Little bit of magic, right there. I think of it as a rune that enables free unlimited parking. Remarkable.

I stepped out of my car and took a lungful of the cold air. Digging through my pockets I came up with three pounds fifty.

I grimaced. This didn't look like the kind of place where it would be easy to find a cheap meal. I'd passed a number of pubs, but they were out of the question: I'd wager my paltry sum would only stretch as far as a pint of coke.

I've been running on near-empty, recently. As I've said, my last business venture did not end well at all. Neither did the one before that, come to think of it. My stock is low and I've not found anything to replenish it with. Cacky love-potions and lucky charms will only get you so far. Gotta have a big-sell. That one speciality item that shines through the crap.

I strolled gloomily along the riverside, carrying my small fold-up table strapped on my back.

The small town had all the markings of an over-priced tourist-trap. I noted the pointless shops as I passed them. Fancy but useless gift emporium, ice-cream parlour, speciality cupcake shop, and even one of those 'trace-your-heritage' outlets that charges for the honour of printing a certificate with your name on it alongside some loosely related coat of arms. Pork pie shop, that one was tempting. It cheered me up, seeing all the trappings of well-dressed tat in the windows. It meant this was a place where I might sell a pretty pendant to a dumb tourist or two.

I realised I had actually reached the famed iron bridge for which the valley was named. It looked all right. Can't say I understand what the fuss is all about, however. A plaque enlightened me with the knowledge that it was the world's first iron bridge. Fancy that.

I felt I ought to be more whelmed by this news, but I suppose I've never been one for sight-seeing. Clearly, I was in the minority. Even on a dreary March morning like this, there were the dedicated anoraks and dog-walkers determined to admire a dull piece of construction history.

Looks like this would be my shop floor for the day, then.

I unfolded my table and rummaged in the large inner pockets of my trench coat. I keep a permanent stash of trinkets about my person at all times. Sparkly crystals, cheap necklaces, colourful rings that will turn your fingers green. Jewellery, it's always the best fall-back in any situation. Never mind if it's slightly cursed.

The first gent to pass me wore a green raincoat. Target acquired.

"Hallo sir. Can I interest you in a souvenir? Genuine piece of Ironbridge heritage."

"Piss off, twat."

"Good day to you," I replied, cheerfully. And they say good manners are dead.

I hummed as I re-arranged my display. Stones and crystals to the front, I think – practically part of the local geology, right? A rock is a rock, wherever it's come from.

"Aren't yow cold,duck?" said a warm voice rich with Shropshire twang. It's a sound halfway between Welsh and Black Country, with 'ow's where 'o's should be. This particular brand of it belonged to the well-tannedand wrinkled face of a lady wrapped in a blue duffel coat. She nodded at me amiably. "Thee'll catch a chill if yow stands out here all day. What're yow sellin'?"

"Dreams," I said, with a dramatic flourish.

"Ow, yeah? Them's nice stones, they are. Is it a necklace?"

"A fine one, indeed.Quality gems, quartz hewn from these very hills. Said to be imbued with the fertile energy of the soil and considered lucky by the bridge-builders of this town." I can create folklore like a pro.

"What bridge builders?" she replied, cocking her head.

I glanced past her to where the imposing, supposedly famous bridge of iron straddled the River Severn.

I changed tack.

"Perhaps you'd be more interested in this ring of polished jasper? An exquisite, earthy gem kept by miners as a token to commune with the ground they tunnelled."

She shrugged her shoulders. "Ow, yeah? Me dad was a miner, he was. He come from Broseley." I sensed I was about to regret my choice of embellishment. "He lived near them squatters in Broseley Wood. Good people they were. But he worked in Jackfield, where there's clay, and that's where me mum come from... Any road, 'im and 'er got married and had me, but he died in the pit when I was little. Ol' Cutty Soames got 'im. Dad used to tell me stories of them what knock in the dark. Then Mum'd tell me stories about Dad, so's I had something to remember 'im by. The Owd Mon, Mum always used to call 'im. Every year she'd light a candle at the window and say: hark at me, Owd Mon. I still loves you but dunna be coming round here expecting your tea anymore. Thee just look after your friends down the pit, an' make sure them nasty knockers dunna come knocking for them. Any road–"

"That's wonderful," I managed to squeeze in. "A great bit of local colour. I commend you to some busybody history society so that you may end your days drowning in nostalgia and tea. Do have a good day, madam."

I gently pushed her away by the elbow and she relented, continuing down the street with a somewhat perplexed expression.

I'm sure the old bag could have talked for England, and would have won the trophy for nonsense to boot. EGS, I call it – Everyone's Grandma Syndrome. Doesn't matter who you are, the afflicted will talk your ear off as if you were their long-lost grandkid, and if you're particularly unprepared they'll start showing you photos, too.

I rubbed my temples and reset my smile. Then I raised my head and scanned the near empty streets for another likely-looking mook. A young lady passed by.

"Hello miss! Take a look at this pendant, shaped like a cup, right? Old Shropshire tale says these were made by fairies to bring-"

"No thanks."

Perseverance, it's key. New target.

"Genuine piece of the Iron Bridge foundation stone? Said to bring good luck!"

"Go away."

"Take a look at this! This rare little crystal was dug out of that very hillside when lightning struck some twenty years ago and they say the whole sky went as green as this gem."

"Who's 'they'?"

"Everyone! Just ask a local."

"Well, it's not that green."

"It's said to bring good luck?"

After three hours I began to tire, and I had sold only one solitary ring. Perhaps bridge-enthusiasts are not my best market. But at least I could now afford one of those enticing pork pies from across the road; their beguiling smell had tortured me all morning.

The woman behind the counter wrinkled her nose when she saw me. I don't always have that effect on women, I swear. But if I was honest with myself, I hadn't paused to wash since leaving Worcester, and my crumpled suit and dishevelled hair weren't doing me any favours.

"One of your finest pork pies, dear lady," I said, flashing a couple of pound coins to prove I wasn't some hobo.

She wrapped my prize in paper and handed it over. "Have a good day," she muttered. "Weirdo."

I decided to savour my pie on the iron bridge itself. Despite the grey sky it was a lovely view of the river. A dense thicket of trees framed the southern bank to my left, with a higgledy-piggledy forest of white and redbrick cottages to my right. This was as close as I had come to being a tourist in years.

It was as I was about to take my first bite that I heard it.

"...heave boys!... y'can rest when it's done... heave!"

It was a tinny sound, as if the voices were coming from a long way off. As I looked about in puzzlement, it continued:

"...ye are all a' igam ogam, get in line!..."

It seemed as if the sound was coming from the other side of the bridge,but it was deserted. I turned in its direction, peering ahead towards an old toll house and empty road beyond.

"...put yer backs into it!..."

And then it clicked. I knew what was going on.

People– that is, most people – think reality is straight forward. It's not.

Reality is fluid, and bulbous. Think of the way a length of cloth might contain the body of a voluptuous woman; the fabric bulges, stretches over the swell of her breast and the curve of her stomach, but creases and crumples under the arms, around the waist. And as she walks, the cloth crumples further, the bulges shift with the bounce of her breasts. Her dress is full of curves and folds and hidden spaces.

From the perspective of Physics, that description is entirely wrong, but it keeps me warm on cold nights.

The point I'm trying to make is that within the folds of reality's dress you can find other bits of reality you didn't know were there. Worlds within worlds. Worlds behind worlds.

And bridges are one way of crossing over to them. The physical bricks and mortar bridges we build in our corner of reality are weak points. Places where the walls separating us from another fold of reality are particularly thin. I'm not sure why.

Despite knowing this, I can't say I've ever heard voices floating across a bridge before.

"Not like that ye bastards... heave..."

Fascinated, I took a step forward. Pure curiosity led me to what I did next.

I began to unfocus.

This is exactly what it sounds like. I exhaled, allowed the edges of myself to blur and fade into the background. In contrast, the world around me became sharper. This is how you travel between those weak spots – you weaken your own space in reality. Ironically, it takes a great deal of concentration to unfocus.

On the other side of the bridge the air shimmered and darkened, collapsing inwards and sucking the road into it. Tendrils of tell-tale fog snaked along the path, pulling my legs forward. I allowed myself to move into the fog and the darkness, colour slipping away behind me. The world blurred as I moved, and suddenly I was on the other side of the bridge.

For a heart-stopping second I thought I'd done it wrong, because the air was still filled with fog.

But as I took in my surroundings I realised it was in fact, more accurately, smog. Thick and yellow and sulphurous, it made my throat prickle.

"Heave! One more ye scummy- eh? Who's this mwnci, then?"

As my eyes adjusted to the stinging air, I made out a gaggle of short shapes in front of me, between two and three feet tall. Hooked noses and pointed ears stuck out from under flat caps, while most else was covered up by grubby shirts and waistcoats and a thick layer of grime.

One, the speaker, held a lantern. He prodded me in the shin.

"Ay, twpsyn. Get out o' the way. We're behind schedule as 'tis."

They were clustered around a large lever dug into the earth at the base of the bridge. I looked back the way I had come, where this end of the iron bridge still stood and fog clouded its other half. It still straddled a river, albeit a rather browner, sludgier one, but the most conspicuous difference was that the space where the North bank and a jumble of redbrick houses should have been, there too, was merely fog. My gaze swung back to the group of goblinesque figures and their massive lever. It looked, against all odds, like they were trying the pry the bridge out of the ground.

"What are you doing?" I asked.

"What business is it o' yourn?"

"I'm just curious. Perhaps I could be of assistance?"

The little hook-nosed creature chuckled croakily. "What assistance could ye be? No miner nor quarryman – soft hands and a softer head. Pen pigyn. Go an' scratch."

There was a ripple of cracked laughter. I smiled brightly. "Maybe you need supplies? How about a hearty delivery of pies and pasties?"

Suddenly I had their rapt attention.

"Go on," said their leader with the lantern. At least, I assumed he was their leader.

"Oh, I can get you anything you want," I said. I brandished my uneaten pork pie for their inspection. Their eyes followed it like starved animals. "Pork pies, steak pies, and definitely some Cornish pasties to remind the boys of home, eh?"

The utter silence was abrupt and disconcerting. Their gazes turned from hungry to menacing.

"Knockers we ain't," said one from the back.

"Nor knackers."

"Nor buccas."

"And definitely not goblins," finished their leader, darkly. "Coblynau we are, and you'd best mind it."

Them what knock in the dark.

I should have paid more attention to that old bat. Of course there would be knockers here, what with all the old mines! And that's what these little wretches so obviously were, though I thought they only lived in Cornwall. But they fit the bill – short stature, grimy clothes, pointy ears and an accent so thick you could spread it on toast. I've done business with the little buggers from time to time, but all I really know about them is that they like their pastries.

The leader beckoned me away and pulled at my coat until I was down to his level.

"Between you an' me, we've been on this job too long. Pasties, Cornish or no, be welcomed. But we'd prefer a good ol' oggie. Y'are right, the boys be missing home."

"Where is home for you, then?" I asked.

"Wales," the non-knocker answered mournfully.

"Welsh knockers! I heard they'd all disappeared. On to pastures new, and such like."

"Ay. We did. Weren't expecting visitors. And it's coblynau, mind." He gave me a pointed look.

"I thought you were called knockers because you knocked on the walls to keep miners safe, or something?"

"That's as may be for them's as is called knockers," he growled. "We be coblynau."

"What are you working on here, anyway?" I said. "It all looks very... industrious."

That was putting it lightly. The smog was only the tip of the polluted iceberg. Dark, angry clouds rumbled by overhead, tracing a line across the red sky to huge chimneys in the distance. I spied other groups of knockers – coblynau, rather – scattered over the barren hillside.

"Doing what we do," he replied. "Mining, quarrying. And most of all, trying to get this gorge closed. Taking an age."

"Why?"

"Some bugger went and put a bloody bridge in the way."

"I mean, why do you want to close it?"

The coblyn looked thoughtful and, I fancy, a tad doubtful.

"'S what we do. Lockin' ourselves in. All agreed on't."

"All except you?"

Now he looked distinctly uncomfortable. "Bridge is a way to get in.Don't need pentwps like yeself comin' in, makin' messes."

I crouched down so that we were eye-to-eye. In my most sincere voice I said, "What about the pasties?"

The little shoulders sank. Under the grime I caught an expression of defeat.

"Would die happy if I 'ad an oggie in my hands. Just can't get the leeks right in this soil. No beef, neither."

 I smiled. "I think we can come to an arrangement." I put an arm round those skinny shoulders. "Tell me, what exactly are you mining here?"