My first thought was, how many people could be supported by five to six boxes of vegetables a day? My second thought, after we’d been walking in a straight line for five hundred metres or so, was this was a hell of a long way to go to the shops. Which was followed by my third thought which was – where would these hypothetical vegetable eaters be getting their protein from? Mushrooms, rats, the occasional commuter? Cannibal navvies – thank you so much Inspector Nightingale.

‘When do you think this was built?’ asked Kumar.

‘The same time as the cut was dug,’ I said. ‘See the way the bricks are laid? That style’s called an English Bond. It matches the work on the tracks and it’s the same kind of London brick. Probably made locally.’

‘They teach you this stuff at Hendon?’ asked Kumar.

‘I had an education before Hendon,’ I said. ‘I was thinking of training to be an architect.’

‘But you were lured away by the glamour of police work,’ said Kumar. ‘Not to mention the high pay and the respect of your peers.’

‘Architecture didn’t work out,’ I said.

‘How come?’

‘I found out I can’t draw,’ I said.

‘Oh,’ said Kumar. ‘I didn’t know that was still a requirement. What with computers and everything.’

‘You still need draughtsmanship,’ I said. ‘Is that a turn ahead?’

Ahead, the passageway curved to the left. Kumar checked his map.

‘We must be following the curve of the trackway. I think you’re right about this being contemporaneous,’ he said. ‘It must have been built by the contractor.’

It made sense. If you’re cutting a nine-metre wide trench through the heart of London you might as well throw in a side tunnel. It could have all sorts of uses, a safety route, a utility conduit. But in that case, why not just make the cut wider? Or if you wanted it covered, why not a colonnade?

‘We should have checked the original plans,’ I said.

‘I did,’ said Kumar. ‘Definitely no secret passages.’

We stopped when we were far enough around the curve to start losing sight of the passage behind us. I flashed my light back towards where Lesley was hopefully standing guard and called her on my airwave.

‘Still here,’ she said, and I saw a flash as she waved her torch at us.

I told her that we might be out of communication soon. Airwave works in the Underground but only when it’s in range of a relay and the tunnels predated digital radio by a good century and a half.

Lesley told us that Nightingale had popped up on the other side of Bayswater Station, which meant that it was growing more likely that James Gallagher could have used the passageway to reach Baker Street. She suggested we look for any evidence that he’d been in our section.

‘Thanks,’ I said. ‘Never would have thought of that.’