From my other pocket I produced the back door key that I’d had for years. With one last quick look back to the street, I eased the door open and let myself in. The cats – Pickle and Maisy – were onto me straightaway, trotting into the kitchen and rubbing around my shins. To them I was just the person who fed them when Alistair and Jennifer were away. My arrival meant food.

‘Not today, ladies,’ I said, shutting the back door behind me. ‘Sorry.’

I went through the house quickly, double-checking all the rooms. I’d watched them all leave this morning but wanted to be sure. The boys were at school and Alistair was at work, none of them due back for hours. I hated myself for what I was about to do, hated the creepiness of skulking around my friends’ empty house. But I had to do it, to protect Lucy. To protect my family.

There were two things that needed to be done.

First, the car.

From the hallway, I went through the connecting door into the garage. In the days after Jake’s hit-and-run, as they corresponded secretly on Messenger, Sean had counselled Jennifer against taking the car in for immediate repairs because he thought it would look too suspicious. Better to wait a few weeks until the lad’s out of hospital and everything’s calmed down a bit. I was grateful, now, for his caution. Jennifer’s blue Volvo was here in the garage, still with minor damage to the front offside wing and the bonnet – dents and scratches in the bodywork where the car had hit Alex Bayley. It looked as if someone had tried to wash it down, but no matter: it was virtually impossible to get rid of every small trace of blood. I took my tools from my bag and chipped away some small flecks of paint from one of the damaged areas, sealing them inside a plastic evidence bag.

Next, I went up to Jake’s bedroom.

Sean had explained it to me. Every file – every document, every video, every email – leaves a unique footprint, a trail. If you have the right tools, and you know what to look for, you can map out the whole lifecycle of a particular file, its whole family tree, including how many copies have been made and where they’ve been sent or saved. He had followed the same process after hacking into Alex Bayley’s accounts to track down and delete copies of the naked video of our daughter.

But the video of her inciting a crime was far more important.