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.

‘Well?’ asked Lesley. I’m better at vestigium than she is and not just because I’ve been apprenticed longer than her.

‘Something’s here,’ I said. ‘You want to make a light?’

Lesley pulled the battery out of her mobile and told Abigail to follow suit.

‘Because,’ I said when the girl hesitated, ‘the magic will destroy the chips if they’re connected up. You don’t have to if you don’t want to. It’s your phone.’

Abigail pulled out last year’s Ericsson, cracked it open with practised ease and removed the battery. I nodded at Lesley – my phone has a manual switch I’d retrofitted with the help of one of my cousins who’s been cracking mobiles since he was twelve.

Lesley held out her hand, said the magic word and conjured a golf-ball-sized globe of light that hovered above her open palm. The magic word in this case was Lux and the colloquial name for the spell is a werelight – it’s the first spell you ever learn. Lesley’s werelight cast a pearly light that threw soft-edged shadows against the tunnel’s concrete walls.

‘Whoa!’ said Abigail. ‘You guys can do magic.’

‘There he is,’ said Lesley.

A young man appeared by the wall. He was white, in his late teens or early twenties with a shock of unnaturally blond hair gelled into spikes. He was dressed in cheap white trainers, jeans and a donkey jacket. He was holding a can of spray paint in his hand and was using it to carefully describe an arc on the concrete. The hiss was barely audible and there was no sign of fresh paint being laid down. When he paused to shake the can the rattling sound was muffled.

Lesley’s werelight dimmed and reddened in colour.

‘Give it some more,’ I told her.

She concentrated and her werelight flared before dimming again. The hiss grew louder and now I could see what it was he was spraying. He’d been ambitious – writing a sentence that started up near the entrance.

‘Be excellent to …’ read Abigail. ‘What’s that supposed to mean?’

I put my fingers to my lips and glanced at Lesley, who tilted her head to show she could keep up the magic all day if need be – not that I was going to let her. I pulled out my standard-issue police notebook and got my pen ready.