‘The roughest of the rough,’ he said.

‘Good,’ I said. ‘We know where to run for help, then.’

The flaring beam from a torch flashed suddenly up through the open hatch, followed by a piercing shepherd’s whistle.

‘That’ll be the patrolman,’ said Kumar and then called down into the dark – ‘David. Up here.’

As Kumar exchanged shouts with the patrolman, Lesley fetched Nightingale. The idea was that he’d keep an eye out on the world above ground and be ready to rush to the rescue or, more likely, pick us up if we surfaced far away.

‘We might as well lower the stairs then,’ I said.

‘If they are stairs,’ said Lesley.

I lay down on the floor-boards and put my head through the hatch, looking for the brass handle to operate the folding staircase. From below a light shone in my face.

‘You might want to stand back a bit,’ I shouted down and the light retreated. I was just reaching for the handle when Lesley spoke in my ear.

‘Are you sure that’s safe?’ she asked.

I looked to find that she’d lain down beside me and had hung her head out the hatch as well.

‘Meaning what?’ I asked.

‘We don’t know what it does,’ said Lesley, looking at the handle. ‘It might swing round and snick your arm right off.’

When me and Lesley were doing our probation at Charing Cross nick I’d learnt to listen to her suggestions – especially after the thing with the dwarf, the show girl and the fur coat.

‘Okay,’ I said. ‘I’ll use a line.’ And scrambled up to find one.

Nightingale waved me aside and muttered something quietly. I felt the forma lining up, a fourth-order spell I thought, with that economy of style and that abrupt twist of strength that I was beginning to recognise as his signare. I heard a creak and a clank which I guessed was the lever pulling itself and then a surprisingly quiet but prolonged rattle of metal as the stairs unfolded and dropped.

‘Or we could do that,’ I said.

‘Was that magic?’ asked Kumar.

‘Can we please get on,’ said Nightingale.

I cautiously put my weight onto the steps, which bounced gently under foot. When it didn’t collapse I walked all the way down. The last step hovered a third of a metre above the rails. A safety measure, I assumed, against electrocution when the track was live. Once they’d seen that I’d made it safely, the others followed me down. Kumar introduced us to a cheerful Welsh geezer called David Lambert – the patrolman. It was his job to walk the line each night checking for faults.

‘I’ve been doing this stretch for six years,’ he said. ‘I always wondered what all that ironwork was for.’

‘You never thought to ask?’ I asked.