Thanks to the national curriculum, even I knew that the Ganges is one of the most sacred rivers in India, although to be honest I couldn’t remember why. Something to do with funeral pyres and chanting. I put it on the list of things that I needed to look into – it was getting to be a long list.

In the end I’d come up with one of those messy compromises. As Brock had written, you couldn’t get the genii locorum to do something as simple as negotiate a contract; symbolism had to be involved. An oath of fealty was out of the question, and a cross-dynastic marriage was too cruel a fate for either Mother or Father Thames. So I suggested an exchange of hostages, a confidence-building measure to cement ties between the two halves of the river; a suitably mediaeval solution designed to appeal to two people who definitely still believed in divine rights. It was a typically English compromise held together by string, sealing wax and the old god network. I’d like to say that I remembered the practice of exchanging hostages from school history classes or from stories of pre-colonial life in Sierra Leone, but the truth was that it came up while playing Dungeons and Dragons when I was 13.

‘Why does it have to be me?’ Beverley had said after she’d found out.

‘It can’t be Tyburn,’ I’d said. You don’t inflict Tyburn on anyone as a gesture of peace and goodwill. And Brent is too young.’ There were other daughters, some the spirits of rivers I’d never heard of and one, a plump, smiley young woman, whose formal name was the Black Ditch. Not that anyone called her that. I figured that Mama Thames thought Beverley was the least likely to cause her embarrassment among the yokels. The hostage from the other side was called Ash, whose river’s principal claim to fame was that it ran past Shepperton Film Studios.

The exchange was scheduled to take place on the evening of 21 June, Midsummer, at Runnymede. Our host was Colne Brook, son Colne who was also the father of Ash – the tributaries of the Thames can get pretty tangled, especially after two thousand years of ‘improvements’. I suspected that the real organisational brains would be Oxley – he wouldn’t want to leave anything to chance. This was confirmed when a series of hand-printed signs appeared beside the road as I negotiated the tricky bit through Hythe End, which guided us neatly down a cul-de-sac lined with semis which terminated in a gate and an impromptu car park.