‘The decimal?’ he said, going to a thick hardback catalogue. ‘Well, unless it’s a common book, there’s no reason why we would have it ourselves. And, in that case, I won’t know which one it’s for.’ I felt deflated. ‘Now, let’s see.’ I handed him the note with the code. He tapped his finger on the first string in the column, DD2261033445298, and counted them with his finger. ‘Thirteen numbers. It can’t be the Dewey at all, then,’ he said. ‘On the Dewey system, three digits will tell you the subject quite precisely. This one, two-two-six, would place a book in’ – he checked down the list in his catalogue – ‘religion; then the Bible; then the Gospels. The following three digits would narrow it down further on the shelf if need be – if you have a lot of books about the Gospels, say – but the rest of the digits in this number wouldn’t mean anything. You can only have six in total. And these letters at the beginning, well … Sometimes you can put letters at the end of the sequence to denote the author’s surname, but not at the beginning.’

I was crushed, I had been sure we had made progress. ‘Are there other systems?’ I asked.

‘None that look like this, I think.’

‘Let’s put the code to one side for a minute,’ Tibbot said, back in our parlour. ‘Let’s say it’s nothing to do with her death. And let’s say that her death wasn’t an accident either. Who might have wanted to do her harm?’

‘I hardly knew her,’ I said. Did anyone really know her?

‘Most killings are just domestic,’ he mused. ‘Someone gets angry, drunk. It’s the boyfriend or husband.’

‘Nick’s not –’

‘I know. Did she have a boyfriend?’

‘I have no idea.’ I wanted to get back to what I was sure was the way in. ‘But this, this code. She was doing something secret, something NatSec wanted to know about. That has to be it, doesn’t it?’

And yet, if she had been involved in subversion, as NatSec’s presence seemed to suggest, what could have brought her to it? Her friend George Orwell had been through one of the re-education camps at Ian Fellowman’s behest. Had there been other friends who had suffered? Perhaps that had hardened her feelings and pushed her to make contact with people who thought as she did. After all, during the War, we had seen acts of extraordinary courage from people one wouldn’t have expected to act that way – housewives who had joined SOE and lived in occupied France with the prospect of Ravensbrück hanging over them; quiet family men who, when the time came, led battalions into the teeth of the German guns. I suppose we all have the capacity within us – it’s only a question of circumstances.