‘On Hathor?’ asked Nightingale. I was impressed; this was some seriously obscure West Coast jazz we were talking about. I complimented him on his jazz erudition. Buying for Lesley had been a pain and in the end I’d settled for a chunky Aran jumper as worn by Danish TV detectives on the verge of a nervous breakdown. Nightingale didn’t ask me what I’d got him, and I didn’t ask what he’d got me.

The night was still and cold as we pulled up outside the fake houses which conveniently served as staging area and changing room. Kumar had brought me a wetsuit and a bright orange overall with yellow reflector patches to go over it. The neoprene was thinner and the fit looser than I was expecting and I wasn’t going to be making any kind of a fashion statement.

‘I don’t expect us to get that wet unless we end up in the drains,’ said Kumar. ‘You want it loose for movement – and you definitely don’t want to overheat.’ He handed me a set of boots that looked like the unfortunate love child of a pair of Doc Martens and a pair of Wellington boots but were surprisingly comfy. We were changing in what everyone had started calling the trapdoor room, with the hatch closed to prevent me falling down it while I hopped about trying to get my boots on.

‘Do we wear our vests?’ I asked.

‘What do you expect to find down there?’ asked Kumar.

‘I honestly don’t know,’ I said.

The Metvest was especially developed for the Met to be both stab and bullet resistant – emphasis on the word ‘resistant’ you notice, not ‘proof’. I’d worn one for two years while in uniform but the last year had got me out of the habit. Still, a Metvest was a comfort in a tight spot, so on they went.

Our helmets were the same high-visibility orange as our overalls and supported state-of-the-art LED headlamps. We divvied up the remainder of the essentials, Kumar got the rope and rescue tools while I took the first aid kit, the emergency food and the water.

‘Damn,’ I said. ‘This is worse than riot training.’

Lesley, who’d been waiting in the next room while we changed, walked in.

‘Nightingale wants to know when you’re going,’ she said.

‘We’re just waiting for the patrolman,’ said Kumar and opened the hatch and stuck his head down to have a look.

‘Are we going to have the place to ourselves?’ I asked.

Kumar climbed to his feet.

‘It’s actually going to be quite crowded down there,’ he said. ‘TfL has every work gang that would take overtime down there tonight. Tomorrow is the last full shopping day before Christmas and it’ll be the first full service day this week – it’s going to be brutal.’

‘Your engineers,’ I said. ‘Are they roughnecks?’