‘Like the ghost,’ I said. ‘Except that you’re not to get on the train tracks, or break into private property or stay out all night, or put yourself in danger in any shape or form.’

‘Can I bunk off school?’ she asked.

‘No, you cannot bunk off school.’

‘I’m not sure I’m really understanding the positive aspects of this arrangement,’ said Abigail.

‘Every Saturday you come down to my office in Russell Square and we go over your notes and we develop action plans based on what you’ve observed,’ I said.

‘That sounds exciting,’ said Abigail.

‘Which will include follow-up investigations and joint field trips to verify any information you bring back.’ I gave her a moment to decode what I’d said. ‘Is that a bit more appealing?’

Nightingale had been horrified by the whole idea when I broached it before coming down to make the pitch.

‘What are you proposing?’ he’d asked. ‘A Girl Guides troop?’

I told him that that was an absurd notion, not least because we’d never satisfy the health and safety requirements for running a troop of Girl Guides. Nightingale said that health and safety was not the point.

‘Think of it as a boxing club,’ I said. ‘You know the boys are going to smack each other in the face anyway, so you might as well channel it into something disciplined. Abigail’s going to be out there looking, so we might as well make use of it, and at least this way we can keep an eye on her.’

Nightingale couldn’t argue with the logic, but he put his foot down on one issue. ‘You are not to teach anybody magic,’ he said. ‘In the first instance you’re far too reckless in who you expose to the art, and in the second you just aren’t qualified to teach. Anyone learning from you is bound to pick up your sloppy form and those embellishments you find so amusing. So I want you to swear now, as my apprentice, that you will not pass on the art to another without my express permission.’

I so swore.

‘If it becomes necessary I will teach Abigail the forms and wisdoms myself,’ he said, and then smiled. ‘Perhaps she’ll prove a more diligent student than yourself in any case.’

Now I watched as Abigail shifted in her seat while she gave the proposition some thought.

‘Do I get badges?’ she asked.

‘What?’

‘Badges,’ she said. ‘You know like in the Guides like Fire Safety and First Aid or Party Planner.’

‘Party Planner – what’s that for?’

‘What do you think it’s for?’

‘Do you want badges?’

Abigail bit her lip. ‘Nah,’ she said. ‘That would be stupid.’

Which was a pity, I thought, badges might be fun, Fireball Proficiency, Werelight, Latin and the ever-popular Fatal Brain Haemorrhage. ‘Do we have a deal or not?’

‘Deal,’ she said, we shook on it and I drove her home.