‘I didn’t know you’re a trainspotter,’ I said.

‘I’m not,’ said Abigail and punched me in the arm. ‘That’s about collecting numbers while this was about verifying a theory.’

‘Did you see the train?’ asked Lesley.

‘No,’ she said. ‘I saw a ghost. Which is why I came looking for Peter.’

I asked where she’d seen the ghost and she showed us the chalk lines she’d drawn.

‘And you’re sure this is where it appeared?’ I asked.

‘He appeared,’ said Abigail. ‘I keep telling you it’s a he.’

‘He’s not here now,’ I said.

‘Course he isn’t,’ said Abigail. ‘If he were here all the time then someone else would have reported him by now.’

It was a good point and I made a mental note to check the reports when I got back to the Folly. I’d found a service room off the mundane library that contained filing cabinets full of papers from before World War Two. Amongst them, notebooks filled with handwritten ghost sightings – as far as I could tell ghost-spotting had been the hobby of choice amongst adolescent wizards-to-be.

‘Did you take a picture?’ asked Lesley.

‘I had my phone ready and everything for the train,’ said Abigail. ‘But by the time I thought of taking a picture he’d gone.’

‘Feel anything?’ Lesley asked me.

There’d been a chill when I’d stepped into the spot where the ghost had stood, a whiff of butane that cut through the fox urine and wet concrete, a Muttley-the-dog snigger and the hollow chest roar of a really big diesel engine.

Magic leaves an imprint on its surroundings. The technical term we use is vestigia. Stone absorbs it best and living things the least. Concrete’s almost as good as stone but even so the traces can be faint and almost indistinguishable from the artefacts of your own imagination. Learning which is which is a key skill if you want to practise magic. The chill was probably the weather and the snigger, real or imagined, originated with Abigail. The smell of propane and the diesel roar hinted at a familiar tragedy.