I sat back and tuned out the doom-mongering warnings of the guy in front. Being on the bus prompted thoughts of the bag Charlie claimed to have left on one just like this. I was now certain that she had been lying. I could picture her rifling through it, spying on my past, feeling sick as she discovered the old photos and letters from ex-girlfriends. Then, in a jealous rage, she had decided to destroy the bag. Dump it in a bin somewhere. She wouldn’t have been able to come up with a story to explain removing just the items relating to my exes; she’d needed to get rid of the whole thing. Then she made up the tale about losing it on a bus, pretended that she had been calling London Transport every day in a desperate bid to find it.

There was, of course, a big difference between the things I knew or strongly believed she’d done – cutting up my photography book, destroying my bag of mementoes – and killing someone. I now knew that she could be jealous, secretive, a liar. But those were things I could deal with, could talk to Charlie about. I didn’t expect her to be perfect. Nobody is.

The crux, I reminded myself, was whether she was jealous, secretive and duplicitous enough to be the one thing that I would never be able to forgive her for. A murderer.

I stared at the filthy streets as they rolled by. I knew where I needed to go next, who I had to talk to.

Harold’s expression changed from puzzlement to delight when he opened his door, his little dog, Dickens, bounding about at his feet.

‘You changed your mind?’ he said.

‘Can I come in?’

‘Yes, please, do. I was just making tea.’

I followed him into what I guessed he would call the sitting room. A fire burned in the hearth and Harold’s dog, Dickens, lay on the rug, chin on paws. The scene reminded me of going to see my grandparents, my mum’s parents, when I was little. They had outlived my parents – I remember them at the funeral, him stoic, her sobbing – but died a few years later within weeks of one another. Couples in my family die in pairs.

Harold came and sat in the armchair opposite mine, putting the tea tray on the table between us and tossing half a biscuit to Dickens, who snatched it up and swallowed it in one gulp.

‘How have you been?’ Harold asked, leaning forward and looking not only directly at me but at the air around me, his eyes roaming about my periphery. It was disconcerting.

‘Not bad.’ I didn’t want to give too much away. ‘I wanted to ask you a couple of questions – about Karen.’

He nodded very slowly. ‘That would be fine. But only if you agree to do something for me.’

I knew what he was going to ask.