She was right. Connected to the footage were links to enhanced pictures of William Skirmish, WITNESS A and the murdering gent in the smurf hat. Contrary to television, there’s an absolute limit to how good a closeup you can extrapolate from an old-fashioned bit of video tape. It doesn’t matter if it’s digital – if the information isn’t there, it isn’t there. Still, someone at the tech lab had done their best, and despite all the faces being blurry it was at least obvious that all three were different people.

‘He’s wearing a mask,’ I said.

‘Now you’re getting desperate,’ said Lesley.

‘Look at that chin and that nose,’ I said. ‘Nobody has a face like that.’

Lesley pointed to a notation attached to the image. ‘Looks like the Murder Team agree with you.’ There was a list of ‘actions’ associated with the evidence file, one of which was to check local costumiers, theatres and fancy-dress shops for masks. It had a very low priority.

‘Aha!’ I said. ‘So it might be the same person.’

‘Who can change their clothes in less than two seconds?’ asked Lesley. ‘Do me a favour.’

All the evidence files are linked, so I checked to see whether the Murder Team had managed to track WITNESS A as he left the crime scene. They hadn’t and, according to the action list, finding him had become a priority. I predicted a press conference and an appeal for witnesses. Police are particularly interested in talking to . . . would be the relevant phrase there.

Smurf Hat had been tracked all the way down New Row, exactly the route Nicholas had said he’d taken, but vanished off the surveillance grid in St Martin’s Lane. According to the ‘action’ list, half the Murder Team were currently scouring the surrounding streets for potential witnesses and clues.

‘No,’ said Lesley, reading my mind.

‘Nicholas …’

‘Nicholas the ghost,’ said Lesley.

‘Nicholas the corporeally challenged,’ I said, ‘was right about the murderer’s approach, the method of attack and cause of death. He was also right about the getaway route, and we don’t have a timeline where WITNESS A is visible at the same time as Smurf Hat.’

‘Smurf Hat?’

‘The murder suspect,’ I said. ‘I need to take this to the Murder Team.’

‘What are you going to say to the SIO?’ asked Lesley. ‘I met a ghost and he said that WITNESS A put on a mask and did it?’

‘No, I’m going to say that I was approached by a potential witness who, despite leaving the scene before I could get his name and address, generated potentially interesting leads that may further the successful outcome of the investigation.’

It made Lesley pause at least. ‘And you think that’ll get you out of the Case Progression Unit?’

‘It’s got to be worth a try,’ I said.