‘There’s something I need to show you,’ I said.

‘Cryptography,’ Tibbot muttered, flicking over the pages of the book I had retrieved from Lorelei’s house. The white box I had found sat beside it on the table; it had meant nothing to him, and I hadn’t let him into my suspicion that it had contained rifle rounds, for fear that he would immediately wash his hands of us. ‘From the Greek krypto, meaning “hidden thing”. It’s NatSec’s department, really, not the police’s.’

I was surprised by his knowledge of Greek. He was a working-class Londoner and not many of them had been to the sort of school that taught Classics. Maybe I had been jumping to conclusions.

‘So do you know anything about it?’

He scratched his white-bristled chin. ‘We’ve had a few pointers in CID. There’ll be a key – a set of numbers or letters. If you have it, it’s easy to decode. If you don’t, you have to look for patterns.’

‘I don’t think we’ve got it.’

‘No.’ We peered at the book again. The strings of letters and numbers varied only occasionally between its twenty-odd sections. ‘Might as well start here,’ Tibbot said, tapping the final section. I slid my finger down the page through the first column of two letters followed by a series of numbers.

Two of the seven strings began with VN. ‘That’s a start,’ I said hopefully. ‘A way in.’

‘Possibly,’ he said. ‘Our best shot, anyhow.’ He didn’t sound very positive.

We tried making phrases from the letters, turning them around and thinking of names for which they could be the initials. But half an hour later we were no further on. ‘What if we’re going at this the wrong way? What if it’s not a code?’ I said.

‘I’ve been thinking about that. They could be identification numbers, say, but for what? Phone numbers are seven digits, including the exchange code. Identity cards have three letters at the beginning of the number.’

‘Bank notes?’ The new decimal currency still felt strange to many of us.

He pulled a pound note from his wallet and examined it. ‘No. Nothing like it.’

‘Map reference?’

‘They’re much shorter.’

We sat reading the numbers backwards and forwards. I saw them spinning in the air, but it did no good. It drove me mad to think that these marks on a page might tell us who was responsible for Lorelei’s death and – more importantly – why it wasn’t Nick. But no matter how much I stared at them, all they did was mock me with their impenetrability.