‘It’s all right,’ I said. ‘Your loss, not mine.’

‘Not that,’ said Lesley, and smacked me on the hand with the flat of her knife. ‘What I said about you being a copper.’

‘Don’t worry,’ I said. ‘I’ve taken your feedback on board, and having extensively workshopped it this morning I now feel that I can pursue my core career-development goals in a diligent, proactive but, above all, creative manner.’

‘What are you planning to do?’

‘I’m going to hack HOLMES to see if my ghost was right,’ I said.

Every police station in the country has at least one HOLMES suite. This is the Home Office Large Major Enquiry System, which allows computer-illiterate coppers to join the late twentieth century. Getting them to join the twenty-first century would be too much to ask for.

Everything related to a major investigation is kept on the system, allowing detectives to cross-reference data and avoid the kind of cock-up that made the hunt for the Yorkshire Ripper such an exemplary operation. The replacement to the old system was due to be called SHERLOCK, but nobody could find the words to make the acronym work so they called it HOLMES 2.

Theoretically you can access HOLMES 2 from a laptop, but the Metropolitan Police likes to keep its personnel tied to fixed terminals — which can’t be left in trains or sold to pawn shops. When a major investigation occurs, the terminals can be transferred from the suite to incident rooms elsewhere in the station. Lesley and I could have sneaked into the HOLMES suite and risked being caught, but I preferred to plug my laptop into a LAN socket in one of the empty incident rooms and work in safety and comfort.