‘Young men are always tempted to use brute force,’ Nightingale had said. ‘It’s like learning to shoot a rifle: because it’s inherently dangerous you teach safety, accuracy and speed – in that order.’

We went through a lot of apples in that first session. I was getting them in the air but sooner or later – splat! There was a brief phase when it was fun and then it got boring. After a week of practice, I could levitate an apple without it exploding nine times out of ten. I wasn’t a happy little wizard, though.

What worried me was where the power was coming from. I never was very good at electricity, so I didn’t know how much power it took to make a werelight. But levitating one small apple against the earth’s gravity – that was essentially the standard definition of one newton of force, and it should be using one theoretical joule of energy every second. The laws of thermodynamics are pretty strict about this sort of thing, and they say that you never get something for nothing. Which meant that that joule was coming from somewhere – but from where? From my brain?

‘So it’s like ESP,’ said Lesley during one of herperiodic visits to the coach house. Officially she was there to liaise with me on the case, but really she was there for the wide-screen TV, takeaways and the unresolved sexual tension. Besides, apart from a couple of unconfirmed cases around the same time as the Neal Street attack, nothing had come to our attention.

‘Like that guy on that show who could move things around,’ she said.

‘It doesn’t feel like I’m moving things around with my mind,’ I said. ‘It’s like I’m making shapes with my mind, which affects something else, which makes stuff happen at the other end. Do you know what a theremin is?’

‘It’s that weirdo sci-fi musical instrument with the loops,’ she said. ‘Right?’

‘Pretty much,’ I said. ‘The point is, it’s the only musical instrument you don’t physically touch. You make shapes with your hands and you get a sound. The shapes are completely abstract, so you have to learn to associate a particular shape with a note and tone before you can get the thing to make a tune.’

‘What does Nightingale say?’

‘He says that if I stopped letting myself get distracted I might spend less time covered in bits of apple.’

At the end of March, the clocks go forward one hour to mark the start of British Summer Time. I woke up late to find the Folly feeling weirdly empty, the chairs in the breakfast room still tucked beneath the tables and the buffet counter unlaid. I found Nightingale reading the previous day’s Telegraph in one of the overstuffed armchairs that lined the first-floor balcony.

‘It’s the change in the clocks,’ he said. ‘Twice a year she takes the day off.’

‘Where does she go?’

Nightingale pointed up towards the attic. ‘I believe she stays in her room.’