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.

I think Mr Punch understood as well, because he fought me all the way as I dragged him across the bridge and in front of the toga-clad officials. These were more echoes from the past, memories trapped in the fabric of the city – they didn’t react when I threw Punch down before them. I was in Year Five when we did Roman history at school, so we didn’t learn a lot of dates but we did do plenty of group work on what it was like to live in Roman Britain. Which was why I could recognise the officiating priest by the purple-striped stole that covered his head. I could also recognise him by his face, although he looked a lot younger than he had when I’d seen him in the flesh. Plus he was clean-shaven and his black hair hung around his shoulders, but it was the same face that I had last seen propping up a fence at the source of the Thames. It was the spirit of the Old Man of the River as a young man.

Suddenly a great many things became clear to me.

‘Tiberius Claudius Verica,’ I called.

Like a man emerging from a daydream, the priest turned his eyes to me. When he saw me he broke into a delighted grin. ‘You must be my gift from the gods,’ he said.

‘Help me, Father Thames,’ I said.

Verica plucked a pilum from the hands of the nearest legionary – the soldier didn’t react – and handed it to me. I smelled freshly cut beech wood and wet iron. I knew what to do. I upended the heavy spear and hesitated. Mr Punch shrieked and bellowed in his strange, reedy high-pitched voice. ‘Isn’t it a pity about pretty pretty Lesley,’ he squealed. ‘Will you still love your pretty little Lesley when her face has fallen off?’