‘Someone pulls the emergency stop between stations you can lose a passenger to a heart attack there and then,’ he said. ‘And you do not want to be evacuating members of the public down a live track – trust me on this.’

You certainly didn’t want to be leaping out on a possibly armed suspect in something shaped exactly like a firing range – especially if you’re going to be the target at the far end.

And the carriages were packed, which took me by surprise, and not with your normal commuters either. Lots of parents with kids, clusters of chattering teenagers, older people in good coats clutching cloth bags or towing shopping trolleys. Last full shopping day before Christmas, I realised, Kumar was right – we really didn’t want to be kicking off anything we couldn’t contain.

It’s a sad fact, but policing would be so much easier if you didn’t have to worry about members of the public getting under your feet.

Kumar had Agent Reynolds, the only one of us who didn’t look like they were remaking Ghostbusters, go ahead and peer through the double set of grimy windows and into the next carriage. When she signalled all clear we opened the connecting doors and stepped through.

There’s no connecting tunnel on a tube train, you open the door and step across the gap to the next carriage. For a moment I was caught in a rush of air and darkness. I swear I heard it then, the whisper, behind the clatter of the wheels and the smell of dust and ozone. Not that I recognised it for what it was – not that I’m sure I know what it was even now.

The Central Line runs what is imaginatively called 1992 Tube Stock consisting of eight carriages. Our suspect was near the front and we were near the back so it took us twelve minutes and five stops to work our way forward. As the train pulled into Oxford Circus we had our suspect, unknown to him, bottled up in the front carriage. So that, of course, is where he chose to get off.

Reynolds spotted him first, signalled back to us and – as he walked past the open doorway where we were standing – we jumped him.

It was as sweet a take-down as anyone could wish for. I got his left arm, Kumar got his right, I slipped my knee behind his, hooked and down he went. We flipped him over on his face and got his arms behind his back.

He wriggled, as sinuously as a fish. It was difficult to keep him pinned. All the while he was completely silent except for a weird hissing sound like a really pissed off cat.

I heard someone in the crowd ask what the fuck was going on.

‘Police,’ said Kumar. ‘Give us some room.’

‘Which one of you has cuffs?’ asked Reynolds.

I looked at Kumar and he looked at me.