That turned out to be Islington, a couple of miles away. We drove past Marks and Spencer and the Vue Cinema, then turned off into a series of streets I had never visited. Another left turn brought us to a surprisingly handsome low-rise building that might have been a council office designed for the more upmarket residents of the borough. My two arresting officers made no comment and there was no sign of any police activity outside. We slowed down and stopped in front of a rather more menacing wall that abutted the building, topped with spikes and razor wire. A gate opened and we drove into a car park filled with police vehicles, gravel, security cameras and despair. As the gate swung shut behind me, I felt utterly cut off from my own life. I can’t quite describe my sense of emptiness, a sense of disbelief that wrenched me from the world I had always known.

A side door led into the custody office, which was small and utilitarian, painted in drab shades of grey and white with official forms pinned to every wall. It reminded me of an old-fashioned bank or building society on a particularly bad day. There were three uniformed officers sitting behind desks with plexiglass screens and computers. I was placed on a stool opposite. But I wasn’t here to take out money. In fact, I was the one being deposited.

‘Name?’ the custody sergeant asked, sweetly. She was in uniform, very neat and well presented, and it struck me that in another life she would have done well as a receptionist, perhaps at the Savoy.

I was about to reply, but then realised she had not expected me to answer for myself. Why should I when I was nothing more than an object to be processed? She had been addressing Cara Grunshaw.

‘This is Anthony Horowitz,’ Grunshaw said. ‘He has been arrested on suspicion of the murder of Harriet Throsby. It is necessary for him to be held in custody in order to interview him and secure evidence.’

They were lines that could have come out of the world’s worst-written play, delivered by actors who had never learned to act. Of all the languages in the world, officialese is the grimmest, lacking any sense of humanity. And the custody sergeant, for all her smiles, was no better. ‘I have heard the reason for the arrest and the need to detain you,’ she told me, once Grunshaw had finished. Her voice mangled the lines, as if she couldn’t quite believe she was saying them. ‘You will be held here to secure and preserve evidence and to obtain evidence by questioning. Is there anything you wish to say at this stage?’

What could I possibly say?

‘I would like to assert and to place on record the possibility that, as evidenced from the two previous statements, you and your colleagues have absolutely no idea what you’re doing. You’re all idiots. This is completely crazy. And if you don’t let me go, I’m going to sue the whole lot of you …’