‘Magically it just makes a lot of “noise”, but the side effect is to dust every microprocessor in about twenty metres.’ I’d gone over the Harrods crime scene that morning with a laser rangefinder and a pocket microscope and found that every chip within ten metres was toast, damaged beyond repair out to twenty, and showing signs of damage out to thirty. I was hoping that further research would reveal it was following the inverse square law, because otherwise I was going to have to call CERN and tell them to take a tea break.

‘I’ve got a similar spell I use to knock out electronic ignitions,’ I said. ‘I call it the car killer.’

‘Oh, that’s a great name,’ said Caroline.

‘Alright,’ I said. ‘What do you call the smoke rope thing that you do?’

She said it had a name in Sanskrit or Bengali or one of those but she just called it the smoke trick.

‘We’ve got a firing range downstairs,’ I said. ‘Want to show me how it’s done?’

*

And it wasn’t as easy as she made it look. I showed her the car killer and the skinny grenade but I kept the water balloons to myself, because a man’s got to have a few secrets. We both managed to do some serious damage to the NATO standard cardboard cutouts and we might have moved on to the paintball gun but the grownups came looking for us and said it was time for Caroline to go home.

I walked them round the corner to where their car was parked. It was an honest to god early model MG MGB, a 1968 judging from the dashboard instruments, although at some point it had been re-sprayed a hideous lime-flower green, once again proving that nine out of ten classic motors are wasted on their owners. As I waved them off I made a note of the car’s index to add to their nominal file.

Nightingale said he’d come to an arrangement with Lady Helena. They would track down Fossman while I worked Operation Marigold to see if I could firm up his involvement from that direction. And, if I could discover who’d supplied the fatal drugs to Christina Chorley and get Tyburn’s daughter off the hook at the same time, so much the better.

‘Just to clarify one thing,’ I said. ‘When you find Fossman, he’s going to be cautioned and interviewed on the record – right?’

‘Do you think that’s wise?’ asked Nightingale. ‘Considering?’

‘Either we’re the law or we’re not,’ I said.

Nightingale nodded gravely and then he looked away and smiled.

‘Agreed,’ he said.

‘Right,’ I said, but the smile worried me.