Throughout the book, the first column of letters and numbers would always be complete, but sometimes the second column would be blank.

I spent a long time trying to guess what it could all mean, but in the end I could make nothing of it other than that Lorelei had had something to hide and had gone to some effort to do so.

I suppose I have never had the sort of mind suited to deciphering codes: I was the very last person you would describe as devious, I just tended to take things at face value. It made me a pushover for my pupils, of course: if they had failed to complete their homework or turned up late, I would believe any excuse if it were told with an honest-looking face.

It would take some puzzling over – and Hazel was waiting at home for me, I reminded myself. So I placed the book back into Lorelei’s bag and set off for home. I would try to decipher it there.

Outside, I kept up a good pace despite the smog and it wasn’t long before I caught sight of our house. It seemed like a sanctuary now, and it was a relief just to come within a hundred paces of it. With Lorelei’s secrets in my possession, Nick might soon be back with us, I told myself.

As I drew close, a beam of light from the front window pierced the mist and I saw my two-year-old ginger cat, Julius, sitting neatly on the step, watching my approach. His amber eyes blinked at me but he jumped up and bounded away when a noise spooked him. I glanced around to see a vehicle coming to a stop behind me: a delivery van with its passenger door open. A man got out, silhouetted by the van’s headlights. ‘Jane Cawson?’ he demanded.

‘Yes,’ I replied, taken aback and worried by the thought of who he might be.

He took me forcefully by the arm and pulled me around to the back of the vehicle. ‘Come on, be a good girl.’

‘Have I done something wrong?’ I asked fearfully. I was just metres from my house but couldn’t reach it.