‘Either you missed him,’ I said. ‘Or he was wounded and swept away.’

‘There’s an access ladder here,’ said Kumar hopefully. It was mounted in a recess just short of the weir.

‘We’re not going to find him tonight,’ I said. ‘We might as well go home.’ I looked at Reynolds. ‘And you’re coming with us for a chat about why you were down here.’

‘I’m going back to my hotel,’ said Reynolds.

‘It’s us or Kittredge,’ I said.

‘It’s all the same to me,’ she said.

‘Children,’ said Kumar. ‘We are leaving.’ He put his foot on his ladder for emphasis.

‘Can you promise me hot towels?’ asked Reynolds.

‘As many as you can eat,’ I said.

‘Okay,’ she said and then she looked past my shoulder. I saw her react and the thought form on her face long before she got her mouth open to yell – behind you!

I lurched around as fast as the water would let me, my mind grasping for the formae, and got the shield up just in time.

The Sten gun is one of those iconic bits of British design, like the Mini or the Tube Map, that has come to represent an era. It’s a submachine gun of very distinctive configuration with its side-mounted magazine and tubular stock. Designed at the start of World War Two to be cheap and cheerful, providing your definition of cheerful was lots of pistol-calibre bullets going in the general direction of the enemy. As Nightingale explained to me, when we found a couple of rusted examples in the armoury, from the individual infantryman’s point of view there really is no such thing as too much personal firepower.

The guy had popped up from nowhere in the small pipe, kneeling to fire in just the same fashion as Reynolds had. My gaze was so fixated on the gun that all I registered was the same pale face, big eyes and a look of terrified determination.

The Sten had a 32-round magazine and early models fired only on full auto. But the action was crude, which meant they weren’t particularly accurate – which is probably what saved my life.

The flash blinded me, the noise deafened me and then a sledgehammer smashed into my chest, once, twice and a third time. I staggered backwards trying to keep my mind focused only on the spell while another part of my mind was yelling that I was dead.

Then the lights went out and I went over backwards and down the weir.

I tumbled, cracking elbow, hip and thigh against the weir’s steps, and then I was dragged face down along the rough bricks of the sewer floor. I pushed myself up and broke the surface gasping for breath. I tried to stand against the current but I’d just made it to my feet when something human-sized smacked into me and sent us both underwater.