‘Women carried on “practising”,’ said Lady Helena, ‘just as they carried on composing, painting and all the other professions from which history has erased them. Mother taught daughter, who passed on the skills through the generations – just as women have always had to do. My mother petitioned the War Office to be allowed to serve. Do you know what they said?’

I admitted that I did not.

‘Nothing,’ said Lady Helena. ‘Not even a polite “piss off.” It’s no wonder she buggered off to Kenya, a new country with new possibilities.’

‘Did she practise in Kenya?’ I asked.

‘No,’ said Lady Helena, ‘Mum was so good at that point that she didn’t have to practise.’

‘And you?’

‘I was mother’s little helper,’ said Lady Helena. ‘We used to find wounded animals and nurse them back to health.’

‘Using magic,’ I asked, trying to be casual.

‘That would be telling,’ said Lady Helena. ‘Although if you really want to know, I’m sure we can come to some sort of arrangement.’

She’d been working towards this, I realised, dangling her information in front of us in the hope that we’d bite. I was willing to bet cash money that she had known all about the Isaacs and the Folly and probably about Nightingale before she so much as set foot in Belgravia nick. We’d been outplayed. But that’s the beauty of being the police – you get to cheat.

‘First you’re going to tell me what your interest in Reynard Fossman is,’ I said. ‘Then I might go to talk my governor about how we’re going to sort this out.’

Lady Helena made an airy gesture.

‘He offered to sell us something,’ she said. ‘An antiquity.’

‘What kind of antiquity?’ I asked. ‘A family heirloom, a genuine Chippendale, a licence to crenelate?’

‘Jonathan Wild’s final ledger.’

‘And why would you want that?’ I asked.

‘You know damned well why,’ she said.

‘Yes,’ I said. ‘But I want you to say it out loud.’

‘The Third Principia,’ said Lady Helena.

‘You’re that keen to turn lead into gold?’

‘Never mind filthy lucre,’ she said. ‘The philosopher’s stone, eternal life and therefore by extension a cure for all that ails you.’ She leant back in her seat and folded her arms. ‘And that’s enough for the starling. You want more, send in the Nightingale.’

‘Do you think there’s more?’ asked Nightingale.

We’d left Lady Helena and the Right Honourable Caroline to stew, on the general police principle of when in doubt keep them waiting. You never know when something incriminating might turn up – it’s happened before, even if you sometimes have to nudge the process along.

Nightingale and Stephanopoulos had made themselves comfortable in her office and sent me off for coffee and biscuits. Once we’d divided up the chocolate hobnobs we got down to the business