To be honest, I’m still not totally sure why I called him. I think I just wanted someone else with me. This is my first ever visit to what could potentially be a time-travelling watch outlet, and I figured it might be nice to have a bit of backup. Or at least a witness.

I point at the posh-looking tinsel-strewn pub across the street. ‘I thought we could get a pint after I’ve sorted my watch,’ I say. ‘Two birds with one stone.’

Harv shrugs and takes another bite. ‘Fair enough.’

We walk up the little staircase and pause in front of the bright purple front door, its gold number 15 glinting in the dying sunlight. On the Tube over here, this seemed like a brilliant idea. But now the doubts are starting to creep in. I mean: what if this is just somebody’s house? How will I even begin to explain what I’m doing here without sounding like a total lunatic?

It’s only the building’s strong Harry Potter vibe that’s keeping my scepticism at bay. If I was a time-travelling watch salesman, this is exactly the kind of place I would live.

Harv looks over at me as I stand dithering on the doorstep. ‘The normal procedure is to knock, man.’

He pounds the door three times with his non-sandwich hand.

For a few seconds, nothing happens. I almost feel relieved. I’m on the verge of suggesting we leave when suddenly I hear muffled footsteps, and the clunk of a heavy lock. And then a familiar scraggly-bearded face emerges.

‘Ah!’ The watch-seller beams. ‘You found me, then!’

A minute later, Harv and I are sitting at the old man’s kitchen table while he makes tea at the stove. And if I thought this place had a strong Potter vibe from the outside, then the inside makes it look positively conventional.

Wooden beams jut out at bizarre angles across the ceiling, and there are clocks covering almost every inch of the walls. They’re all different shapes, styles, colours and sizes, and their combined ticking sounds like the hooves of a hundred tiny stampeding horses. Cogs and watch faces and straps and springs are scattered across the kitchen counters, nestled among dozens of notebooks and pieces of paper, all of them teeming with strange diagrams and illegible scrawls. Next to the old-fashioned stove, there’s a sink big enough to bathe a St Bernard in, which sends out a howling clanking sound as the old man spins its rusty faucets. He is still wearing the same ill-fitting blue suit and reindeer tie, and his unruly copper-grey hair flaps wildly as he flits about taking mugs out of the cupboards. There’s something so familiar about him.

Harv is gazing around the room, looking understandably bemused. ‘So you … sell watches, do you?’ he asks.

‘Among other things.’ The old man chuckles. ‘In fact, I was selling chestnuts fairly recently.’