There was once a gallows, close to where Marble Arch now stands, where they used to hang the criminals of old London town. The gallows was named after the village, whose inhabitants profited so greatly from the grisly spectacles that they built viewing stands to bring in the punters, which was named after the river that ran through it. The river was named the Tyburn. They hanged poor Elizabeth Barton there and Gentleman Jack, for all that he’d escaped four times before, and the Reverend James Hackman for the murder of pretty Martha Ray. I knew all this because after Beverley’d dropped her sister’s name into the conversation as the one who knows people who matter, I made a point of finding out.

‘I thought it was time you and I had a little chat,’ said Tyburn.

I offered her the flowers, which she took with a delighted laugh. She pulled my head down and kissed me on the cheek. She smelled of cigars and new car seats, horses and furniture polish, Stilton, Belgian chocolate and, behind it all, the hemp and the crowd and the last drop into oblivion.

I’d traced the sources, as well as I could anyway, of all the lost rivers of London. Some, like the Beverley Brook, the Lea or the Fleet were easy to find, but the location of the Tyburn, the legendary Shepherd’s Well, had got lost in the mad Victorian steam-powered expansion of London in the latter half of the nineteenth century. This fountain was obviously at the source, but the fountain itself, I suspected, had been looted by an enterprising official in the last days of the Empire.

I was thirsty – I would have liked a drink.

‘What would you like to talk about?’ I asked.

‘For a start,’ said Tyburn, ‘I’d like to know what your intentions are with regards to my sister.’

‘My intentions?’ I asked. My mouth was very dry. ‘My intentions are purely honourable.’

‘Really?’ she said, and crouched down to retrieve a vase from behind the fountain. ‘Is that why you took her to see the pikeys?’

Pikey is not a word a well brought-up young policeman is supposed to use. ‘That was just a preliminary, exploratory investigation,’ I said. ‘And Oxley and Isis are not pikeys.’

Tyburn drew the back of her hand down the back of the marble water carrier, and the trickle from the gourd thickened into a strong stream from which she filled the vase. ‘Still,’ she said as she unwrapped the roses, ‘not the sort of people one wants one’s sister associating with.’

‘We don’t get to choose our family,’ I said cheerfully. ‘Thank God we can choose our friends.’