I’d expected the inside to resemble the back of a stage set, but instead we found ourselves in a classic Victorian hallway complete with a badly scuffed black and white tiled floor and yellow wallpaper that had faded to a pale lemon. The only real difference was that instead of running front to back it ran side to side, linking both of the notional terraces. On our left there was a duplicate front door and ordinary interior doors at each end.

I went left. Lesley went right.

Beyond my door was a room with bay windows, net curtains and bare floorboards. There was a smell of dust and machine oil. I spotted something green on the floor and retrieved a lettuce leaf – still crisp. The back wall was plastered, grubby and devoid of windows. It was a locked-room mystery – the case of the missing vegetables. I was just about to go and see if Lesley had had more luck when I noticed that a black iron ring had been inset into a floorboard. A closer inspection revealed that it was the handle for a trap door and, with a surprisingly easy lift, it opened to reveal a six-metre drop onto the tracks below. Carefully I lay down on the floor and stuck my head through the hatchway.

I was disconcerted to see that the two half-houses were held up by a series of wooden beams. They were old, black with soot and spanned the width of the trackway, bolstered at the ends with diagonal beams that had been fitted into the brick walls of the cut. Attached with iron bolts to the nearest beam was a long flattened contraption made of iron, dark-coloured wood and brass. It took me a bit of squinting but I finally realised that it was a staircase in the manner of a folding fire escape neatly concertinaed and stowed to the underside of the house.

Within easy reach of the hatch was a brass and leather lever with a clutch handle like those you find on vintage cars and steam engines. I reached out to see if it would move.

‘What’s down there?’

I turned my head to find Lesley staring down at me.

‘A folding staircase I think,’ I said. ‘I’m just going to see if I can unlock it. It should drop straight down onto the tracks.’

I reached once more for the lever, but as I did so a Circle Line train clattered directly beneath me on its way to Bayswater Station. It took about thirty seconds to go past.

‘Are you sure that’s a good idea?’ asked Lesley.

‘I think,’ I said slowly, ‘it would be better if we call BTP first. What do you think?’

‘I think you may be right,’ said Lesley.

So I got to my feet, closed the hatch and called Sergeant Kumar.