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?’

This is not a person, I told myself, and drove the pilum into Mr Punch’s chest. There was no blood, but I felt the shock as it pierced skin, muscle and finally the wooden planking of the bridge itself. The revenant spirit of riot and rebellion was pinned like a butterfly in its display case.

And people say modern education is a waste of time.

‘I asked the river to give us a sacrifice,’ said Tiberius Claudius Verica, ‘and a sacrifice was provided.’

‘I thought the Romans frowned on human sacrifice,’ I said.

Verica laughed. ‘The Romans haven’t arrived yet,’ he said.

I looked around. He was right, there was no trace of London – or the bridge. For a moment I hung suspended like a cartoon character, and then I fell into the river. The Thames was cold and as fresh as any mountain stream.