‘De Veil?’ asked Mr Punch. ‘Yes please, I’m sure he’ll be delicious.’

Revenant, spirit of riot and rebellion, I thought, you idiot. He eats ghosts. I needed something stronger. Brock had written that the genii locorum, the gods and spirits of place, were stronger than ghosts. Was there a god of justice? And where would I find him – or maybe her? Then I remembered: a statue of a woman stands atop the dome of the Old Bailey. In one hand she holds a sword and a set of scales. I didn’t know if there was a goddess of justice or not, but I was willing to bet big money that Mr Punch would know.

‘Why don’t we go and ask the nice lady of the Old Bailey?’ I suggested.

He tensed, and I knew I’d bet right. He struggled again and slammed his head back, aiming for my chin, but that isn’t exactly a new one for a policeman, so I had my head back safely out of reach.

‘You’re going up the steps this time,’ I said.

Mr Punch went limp, defeated I thought, but then he began to shake in my grip. At first I thought he was crying, and then I realised it was laughter. ‘You’re going to find that a bit difficult,’ he said. ‘You seem to have run out of city.’

I looked around and saw that he was right. We’d gone back too far, and now there was nothing left of London but huts and the wooden stake rampart of the Roman camp to the north. There was no stonework at all, nothing but the new-cut smell of oak planking and hot pitch. Only one thing stood complete – the bridge. It was less than a hundred metres away and constructed of square-cut timbers. It looked more like a fishing pier that had got ideas above its station and crossed the river in a fit of exuberance.

I could see a crowd halfway across, sunlight flashing off the brass fittings of a file of legionnaires standing at attention. Beyond them a cluster of civilians in togas chalked to a blinding white for a special occasion and watching a couple of dozen men, women and children in barbaric trousers and brass torcs.

Suddenly I understood what it was Mama Thames had been trying to tell me.