We headed out through the exit into Holland Park station – at a walking pace, thank god. Reynolds had been shadowing him and we found her crouched at the bottom of a flight of stairs trying to get an angle on anyone at the top without being seen.

‘He just went up,’ she whispered to us.

I asked if she was sure it was him.

‘Pale face, big eyes, that weird round-shouldered posture,’ she said. ‘Definitely him.’

I was impressed. I hadn’t even noticed his posture. The sisters had said that after the stairs there was a short corridor and then a fire door out into the station proper. We reckoned he’d hear our boots if we ran up behind him. So we walked up, having a casual conversation in the hope we’d sound like weary clubbers. In the course of the first two flights I learnt that Special Agent Kimberley Reynolds was from Enid, Oklahoma and had gone to university at Stillwater and thence to Quantico.

Sergeant Kumar turned out to be from Hounslow and had studied Engineering at Sussex University but had fallen into policing. ‘I’d have been a terrible engineer,’ he said. ‘No patience.’

I had a jazz anecdote about my father all ready to go when we very clearly heard the sound of a door slamming shut up ahead – at which point we legged it.

It was an ordinary fire door, heavily spring-loaded, presumably so Olympia and Chelsea’s friends could leave without letting the commuter traffic leak back in. We went through it slowly and quietly and found ourselves in an alcove tucked away near the stations lifts. Our suspect wasn’t amongst the passengers waiting to go up in the lift and, according to them, they’d been waiting at least a couple of minutes, which was too long for him to have gone up earlier.

‘Stairs or platforms?’ asked Kumar.

‘He likes to stay underground,’ I said. ‘Platforms first.’

We caught a break when I spotted him through the grilled windows where the corridor cut across the top of the eastbound platform. We ran as quietly as we could down the next flight of stairs and piled up like cartoon characters at the entrance to the platform. I was just nerving myself to have a look around the corner when Kumar pointed at the convex mirror at head height opposite. This was a holdover from the days before CCTV when station staff and BTP had to scope out stations with the mark one eyeball.

I spotted him, small and oddly shaped in the mirror, at the far end of the platform.

‘If he’s still armed,’ said Kumar. ‘We’ll never get close.’

I felt a puff of air on my face and the rails began to sing. It was too late – a train was coming.

21. Oxford Circus

21

Oxford Circus

Sergeant Kumar was very clear about one thing – you don’t do shit when the train is in motion.