‘Now the most popular music venue in Europe,’ she said. ‘The Rhine Maidens come to visit me to see how it’s done.’ She gave me a significant look, and I wondered who the hell the Rhine Maidens were.

‘Perhaps Father Thames sees things differently,’ I said.

‘Baba Thames,’ spat Mama. ‘When he was a young man he stood where I stood, on the bridge, and made the same promise I did. But he hasn’t been below Teddington Lock since the Great Stink of 1858. He never came back, not even after Bazalgette put the sewers in. Not even for the Blitz, not even when the city was burning. And now he says this is his river.’

Mama Thames pulled herself upright in her chair as if posing for a formal portrait.

‘I am not greedy,’ she said. ‘Let him have Henley, Oxford and Staines. I shall have London, and the gifts of all the world at my feet.’

‘We can’t have your people fighting each other,’ I said. The ‘royal we’ is very important in police work; it reminds the person you’re talking to that behind you stands the mighty institution that is the Metropolitan Police, robed in the full majesty of the law and capable, in manpower terms, of invading a small country. You only hope when you’re using that term that the whole edifice is currently facing in the same direction as you are.

‘It’s Baba Thames who is trespassing below the lock,’ said Mama Thames. ‘I am not the one that needs to back off.’

‘We’ll be the ones that talk to Father Thames,’ I said. ‘We expect you to keep your people under control.’

Mama Thames tilted her head to one side and gave me a long, slow look. ‘I’ll tell you what,’ she said. ‘I’ll give you until the Chelsea Flower Show to bring Baba to his senses; after that, we shall take matters into our own hands.’ Her use of the ‘royal we’ was a great deal less tentative than mine.

The interview was over, we exchanged pleasantries and then Beverley Brook showed me to the door. As we got to the atrium she deliberately let her hip graze mine, and I felt a sudden hot flush that had nothing to do with the central heating.

She gave me an arch little look as she opened the door for me.

‘Bye-bye, Peter,’ she said. ‘See you around.’

When I got back to the Folly I found Nightingale in the reading room on the first floor. This was a scattering of upholstered green leather armchairs, footstools and side tables. Glass-fronted mahogany bookcases lined two walls, but Nightingale had admitted to me that in the old days people had generally come here for a nap after lunch. He was doing the Telegraph crossword.

He looked up as I sat down opposite. ‘What did you think?’