I struck out for the south bank on the basis that the Thames path ran on that side so there were more likely to be members of the public able to render assistance. Plus the OxoTower made a convenient landmark. I didn’t try to fight the current and concentrated the last of my strength on getting closer to the bank.

I’ve never been what you’d call a strong swimmer but if the alternative is being a statistic it’s amazing what you can pull out of the reserves. The world contracted around me until there was nothing but the cold weight of my wet clothes, the pain in my arms, and the occasional malicious slap in the face by a wave that would leave me gasping and spitting.

Mama Thames, I prayed. You owe me, get me to shore.

I realized suddenly that my arms weren’t really working properly and that it was getting harder just to keep my face above the water.

Mama Thames, I prayed again. Please.

At some point the tide turned and I found myself being washed back upstream until a random eddy caught me and gently shoved me onto the dirty mud of the Thames bank. I pulled myself slithering as far up the foreshore as I could manage before rolling onto my back. I stared up at the rain clouds above, lit a dull sodium red by the lights of the city, and thought that of the many things I never wanted to do again this was near the top. I was so cold that my fingers and toes had gone numb, but I was shivering, which I took to be a good sign because I had this vague notion that it’s when you stop shivering that you should be really worried. I decided that I could afford to stay where I was and catch my breath or maybe some sleep—it had been a long day.

Contrary to what you might have been told, it is almost impossible to lie prostrate and groaning in a public place in London without attracting a crowd of putative good Samaritans—even when it’s raining.

“Are you all right, mate?”

There were people on the parapet above me. I looked at their quizzical upside-down faces from where I lay. Helpful people with mobile phones who would helpfully phone the police who in turn would probably ask me to help them with their inquiries about a certain hijacked ambulance.

Do not meddle in the affairs of wizards, I thought. For they are soggy and hard to light.

I considered making a run for it but the paramedic and the ambulance driver could both identify me and in any case I was just too knackered to move.

“You just hold on, mate,” said the voice from above. “The police are on their way.”