On the counter in front of me was a cutlass used by the Kray brothers. Next door was a rocket launcher used by the IRA. At first I thought Caine had a mini-kitchen in here, but it turned out to be the cooking pot where serial killer Dennis Nilsen boiled the meat off his victims before pouring it down the drains. And there were more knives than I had seen in the basement of West End Central.

The Black Museum was spread over several large, neat rooms full of glass display cases and exhibits and shelves with the facemasks of men who stole lives. Blank, ordinary, banal-looking men who shot, poisoned, stabbed, chopped up, boiled and ate their victims. All these pathetic little men who had abruptly aborted the happiness of countless lifetimes, all these savage creeps who had built a mountain of human misery.

Yes, I had been here before.

But this time was different.

Now I was not with my peers.

There was no hiding in a crowd, and no easy laughter to relieve the tension. This did not feel like a school trip. This time the Black Museum confronted me with all its horror, and its collection of human cruelty, and it was just too much for me.

Or perhaps it was something else. The first time I had been here, on that visit with the Crime Academy, I was a cocky unmarried kid who knew nothing about loss. And now I knew.

First came the sweat, and then suddenly I was crouching over a wastepaper basket, quietly being sick. Mallory and Sergeant Caine came into the room. If they saw my discomfort then they gave no sign.

‘Your weapon could be custom-made,’ Caine was saying. ‘Some kind of knife that’s made to cut a man’s throat – and only that. Something that’s made for that purpose and that purpose alone.’

Outside, Big Ben was chiming six. Mrs Murphy would have picked up Scout and would be making their dinner while my daughter and our dog chased each other across the great open space of our loft.

I stood up, looking with embarrassment and disgust at the yellow bile I had brought up. Mallory lightly patted my shoulder.

I couldn’t look at him just yet.

‘Nice cup of tea?’ he said.

Mallory took me to his home.

His wife appeared at the door as we pulled up outside a terraced house on a quiet street in Pimlico. A tall, slim, grey-haired woman with an amused twinkle in her eye and carpet slippers on her feet. A West Highland White Terrier stood watch between them as Mr and Mrs Mallory communicated in the shorthand of the long-term married.

‘Done for the day then?’ she said, folding her arms.

‘Not quite, hen,’ he said, pecking her cheek.

‘Going out again then.’

‘Spruce up a bit first.’

‘Something to eat?’

‘Tea would be lovely.’