Kumar used his torch to pick out a set of modern steel doors set in a decidedly Victorian brick arch. ‘I thought he might have got in via the old works access but they’re still sealed – you might want to fingerprint them though.’

‘Where are we now?’

‘Under Marylebone Road heading east,’ said Kumar. ‘There’s a couple of old ventilation shafts further up I want to check. Coming?’

It was seven hundred metres to Great Portland Street, the next station. We didn’t go the whole way, just until we could see the platform. Kumar checked his access points and said that had our mystery boy got off the platform there he would have been spotted by the ever-vigilant CCTV operators.

‘Where the fuck did he get on the tracks?’ said Kumar.

‘Maybe there’s some other way of getting in,’ I said. ‘Something that’s not on the blueprints, something we missed.’

‘I’m going to get the regular patrolman down here,’ said Kumar. ‘He’ll know.’ Patrolmen spent their nights walking the tunnels looking for defects and were, according to Kumar, guardians of the secret knowledge of the Underground. ‘Or something,’ he said.

I left Kumar waiting on his native guide and headed back towards Baker Street. I was halfway there when I slipped over a loose bit of ballast and fell on my face. I threw out my hands to break my fall, as you do, and it didn’t escape my attention that my left palm had come slap down on the electrified middle rail. Crispy fried policeman – lovely.

I was sweating by the time I climbed back onto the platform. I wiped my face and discovered a thin coating of grime on my cheeks – my hands were black with it. Dust from the ballast, I guessed. Or maybe ancient soot from when steam locomotives pulled upholstered cars full of respectable Victorians through the tunnels.

‘For god’s sake somebody get that boy a hanky,’ said a large voice with a Northern accent. ‘And then someone can fucking tell me why he’s here.’

Detective Chief Inspector Seawoll was a big man from a small town outside Manchester. The kind of place, Stephanopoulos had once said, that explained Morrissey’s cheery attitude to life. We’d worked together before – he’d tried to hang me on stage at the Royal Opera House and I’d stuck him with 5cc of elephant tranquilliser – it all made sense at the time, trust me. I’d have said that we came out about even, except he had to do four months of medical leave which most self-respecting coppers would have considered a bonus.

Medical leave was obviously over and Seawoll was back in charge of his Murder Investigation Team. He’d taken a position up the platform where he could keep an eye on the forensics without having to change out of his camelhair coat and handmade Tim Little shoes. He beckoned me and Stephanopoulos over.