After leaving Portico, by a different exit from the one I had used the previous night, I spotted a shiny green Ford Escort slide out into the traffic. There was only a driver in the car; no one else. I turned down to the Mall, around Buckingham Palace and up towards Hyde Park Corner. The Escort sat well back. More for amusement than anything else, I turned from Hyde Park Corner into St George’s Street and drove into Belgrave Square, where I suddenly pulled over sharply to the left to a florist stand. The Escort was caught completely on the hop. I left him to circle several times around Belgrave Square, while I made a slow and ponderous attempt to decide which bunch to buy for my sick aunt in Maidenhead, before telling the not very amused vendor that I had just remembered she was allergic to flowers, and then drove off.

I decided I would take the Escort for a drive down Walton Street. Walton Street is a smart, narrow street, lined with restaurants, art galleries, and precious little shops staffed by horsey ladies who talk to each other as if they are shouting from distant lavatory seats.

The doyen of the Walton Street restaurants is Walton’s, which once had the dubious distinction of being blown up by the IRA. In front of this restaurant, the road hooks sharply to the right and comes to a traffic light. It is rare to find an occasion, day or night, when there isn’t a bottleneck in Walton Street and today, fortunately, was no exception. I ground to a halt in the jam, about fifty yards back from Walton’s, with the Escort four cars behind me. I pulled the handbrake on, then, ducking my head, I slid across and climbed out the passenger door.

Crouched right down, and ignoring the curious gaze of the driver in the van behind me, I crept down the side of the van, and down the side of the two cars behind it. As I reached the Escort, out of the driver’s line of vision, with one hand I unholstered my Beretta from an inside pocket – clicking off the safety catch in the process – and with the other hand I opened the Escort’s door.

‘Good morning,’ I said, climbing in.