‘He’s eating me,’ said Nicholas, and then his face was sucked backwards into the wall. For a moment I felt a strange tugging sensation on the back of my head and threw myself backwards. Toby barked once, and then turned and shot off in the direction of Russell Square. I landed heavily on my back, which hurt, so I lay there feeling stupid for a moment and then got back on my feet. Cautiously, I approached the church and gingerly laid my palm on the stone again.

It felt cold and rough, and there was nothing else. It was if the vestigia had been sucked out of the stones the same way it had been back at the vampire house. I snatched my hand away and backed off. The Piazza was dark and quiet. I turned and strode into the night, looking out for Toby as I went.

He’d run all the way back to the Folly. I found him in the kitchen curled up in Molly’s lap. She comforted the dog and gave me a stern look.

‘He’s supposed to face danger,’ I said. ‘If he stays, he works.’

Just because I had an active case didn’t mean I was excused practice. I’d persuaded Nightingale to show me the fireball spell, which was, not surprisingly, a variation on Lux, with Iactus to move it about. Once Nightingale was convinced I could do the first part without burning my hand off, we went down to the firing range in the basement to practise. Not that I had known we had a firing range until then. At the bottom of the back stairs you turned left instead of right, through a set of reinforced doors that I’d always assumed was a coal store, and into a room fifty metres long with a wall of sandbags at one end and a line of metal lockers at the other. A row of vintage Brodie helmets hung from pegs above a line of khaki gasmask cases. There was a poster, white lettering on a blood-red background, that said: ‘Keep Calm and Carry On’, which I thought was good advice. There was a stack of cardboard silhouettes at the target end, brittle with age but still discernible as German soldiers with coal-scuttle helmets and fixed bayonets. Under Nightingale’s direction I set up a row of them against the sandbags and trotted back to the firing line. Before we started, I checked to make sure I wasn’t carrying my brand new mobile phone.

‘Watch carefully,’ said Nightingale. Then he flung out his hand, there was a flash, a sound like a sheet being ripped in half, and the target on the extreme left-hand side was blown into flaming fragments.

I turned at the sound of excited clapping, and found Molly hissing with delight and standing on tiptoe like a small child at the circus.

‘You didn’t say the Latin,’ I pointed out.