He never really explained what part I played in the bigger picture. That was Walt’s way. He was the only person who could ever see the whole playing field at once. All he asked was that when a certain call came, I be there to listen.

‘No need to bother those worry-waters at GCHQ,’ he said, sitting on the edge of my desk in a shabby blue suit and with a drop of moisture threatening to fall from the end of his nose. He was tanned and there was some nasty rash beneath the thinning strands of hair across his pate. He looked older and sicker than when we had last worked together. His ebullience had gone. He wasn’t trying to make me laugh. He stank of drink and American cigarettes. But he was a legend in the Service and he rated me highly. So I did as I was bid.

‘A useful source will be making a transmission at a pre-appointed time on a pre-appointed frequency. The frequency in question will be provided for you when required. So too the radio receiver. It might be in the next week, it might be a month from now. I’m sorry it’s so vague. You are to listen in, make the recording and provide the transcript. You are to give it to me in person, or nobody at all. You place the recording where I tell you. If I never ask you for the transcript, you destroy it. If anybody else asks you for it, you destroy it. Only if I ask, and reference a Mrs Tenebrae when so doing, do you provide it for me. Do you understand?’

It wasn’t that outlandish a request. I’d been tasked with doing stranger and more dangerous things. It felt good to be trusted with something that was clearly important to him, even if it bent some of the very protocols that Walt had first established. I remained on high alert. Eight days later, I came back from a brief leg-stretch to find a copy of Tenebrae by Ernest G Henham on my desk. It had a library ticket in the title page. It was due back the following day. Recognizing Walt’s favoured technique, I took the book back to the little library in Wandsworth. The librarian looked as though she was trying to chew a lemon with her buttocks: all sour and pinched and puckered up. But when she stamped the book she handed me another and told me there was a reference section where I might like to go and read. Out of sight, I ran my finger down the binding and found the little scrap of paper that one of Walt’s men had left for me. It gave me the time, date and radio frequency. The little collection of line drawings on the flip side weren’t hard to work out. They marked out the O’Farrell mausoleum. Highgate Cemetery.