‘She certainly thinks she’s the Goddess of the Thames,’ I said. ‘Is she?’

‘That’s not a terribly useful question,’ said Nightingale.

Molly silently arrived with coffee and a plate of custard creams. I looked at the biscuits and gave her a suspicious glance, but she was as unreadable as ever.

‘In that case,’ I said, ‘where does their power come from?’

‘That’s a much better question,’ said Nightingale. ‘There are several conflicting theories about that; that the power comes from the belief of their followers, from the locality itself or from a divine source beyond the mortal realm.’

‘What did Isaac think?’

‘Sir Isaac,’ said Nightingale, ‘had a bit of a blind spot when it came to divinity – he even questioned whether Jesus Christ was truly divine. Didn’t like the idea of the Trinity.’

‘Why was that?’

‘He had a very tidy mind,’ said Nightingale.

‘Does the power come from the same place as magic?’ I asked.

‘All of this will be much easier to explain once you’ve mastered your first spell,’ he said. ‘I believe you could get a good two hours of practice in before afternoon tea.’

I slunk off in the direction of the lab.

I dreamed that I was sharing my bed with Lesley May and Beverley Brook, both lithe and naked on either side of me, but it wasn’t nearly as erotic as it should have been because I didn’t dare embrace one for fear that I’d mortally offend the other. I had just devised a strategy to get my arms around both at the same time when Beverley sank her teeth into my wrist and I woke with a terrible cramp in my right arm.

It was bad enough to make me fall out of bed and thrash around being uselessly stoic for a good two minutes. There’s nothing like excruciating pain for waking you up, so once it was clear I wasn’t going back to sleep I left my room and went looking for a snack. The basement of the Folly was a warren of rooms left over from when it boasted dozens of staff but I knew that the back stairs bottomed out next to the kitchen. Not wanting to disturb Molly, I padded down the steps as quietly as I could but as I reached the basement, I saw that the kitchen lights were on. As I got closer I heard Toby growl, then bark and then there was a strange rhythmic hissing sound. A good copper knows when not to announce his presence, so I crept to the kitchen door and peered in.