She stared at me in amazement and then her tiny mouth spread into a wide smile.

‘Oh my god,’ she said. ‘You’re one of them – aren’t you?’

‘May I come in?’ I asked.

‘Yeah, yeah,’ she said. ‘Fuck me – the Folly.’

She was still shaking her head as she ushered me into a stone-paved entrance hall – dim and cool after the summer sun – then into a half-oval sitting room smelling of potpourri and warm dust and back out via the middle of three French windows.

The window opened onto a series of landscaped terraces that descended down towards more woods. The garden was informal to the point of being chaotic, with no organised beds. Instead, clumps of flowers and flowering bushes were scattered in random patches of purple and yellow across the terraces.

Mellissa led me down a flight of steps to a lower terrace where a white enamelled wrought-iron garden table supported a bedraggled mint-green parasol shading matching white chairs, one of which was occupied by a thin grey-haired man. He sat with his hands folded in his lap, staring out over the garden.

Anyone can do magic, just like anyone can play the violin. All it takes is patience, hard graft and somebody to teach you. The reason more people don’t practise the forms and wisdoms, as Nightingale calls them, these days is because there are damn few teachers left in the country. The reason you need a teacher, beyond helping you identify vestigium – which is a whole different thing – is because if you’re not taught well you can easily give yourself a stroke or a fatal aneurism. Dr Walid, our crypto-pathologist and unofficial chief medical officer has a couple of brains in a jar he can whip out and show you if you’re sceptical.

So, like the violin, it is possible to learn magic by trial and error. Only unlike potential fiddlers, who merely risk alienating their neighbours, potential wizards tend to drop dead before they get very far. Knowing your limits is not an aspiration in magic – it’s a survival strategy.

As Mellissa called her granddad’s name I realised that this was the first officially sanctioned wizard, apart from Nightingale, I’d ever met.

‘The police are here to see you,’ Mellissa told him.

‘The police?’ asked Hugh Oswald without taking his eyes off the view. ‘Whatever for?’

‘He’s up from London,’ she said. ‘Especially to see you.’ Stressing the especially.

‘London?’ said Hugh, twisting in his chair to look at us. ‘From the Folly?’

‘Yes, sir,’ I said.

He climbed to his feet. He’d never been a big man, I guessed, but age had pared him down so that even his modern check shirt and slacks couldn’t disguise how thin his arms and legs were. His face was narrow, pinched around the mouth, and his eyes were sunken and a dark blue.

‘Hugh Oswald,’ he said holding out his hand.