‘Isobel Sturridge is Faith,’ I say. ‘Even if she isn’t Billy, she’s Faith Kendell.’

Charlie nods. ‘That’s … yes.’

Of course she was never going to tell me. Why should she?

‘I did a bit of digging around online,’ I say. ‘I think I’ve got something on the Liv and Gibbs front.’

Charlie grabs my wrist. ‘You’d better not be joking.’

‘I’m not. I’ve found Natalie – Nikhil’s girlfriend. Her surname’s Burge. She has a long job title: Director of Science and Plants for Schools at Cambridge University’s Botanic Garden. She’s on Twitter – not as herself; as Science and Plants or some such – and she’s going for a wedding dress fitting tomorrow afternoon in Cambridge city centre at a place called The Tailor’s Cat. It’s a bridal boutique. Guess who’s also going?’

‘I’ve no idea.’

‘Charlie. You used to be a detective.’

‘Liv’s going too? How do you know?’

‘From Natalie’s tweet last night: “So excited about wedding dress fitting on Tuesday at The Tailor’s Cat” followed by Liv’s Twitter handle. They hadn’t been tweeting each other, there was no conversation – just that one tweet with Liv included. And – I think this is strange, I don’t know if you will – they don’t follow each other on Twitter. They’ve never communicated there before. Natalie’s tweets, all apart from that one, are “Picture of rare monkey puzzle tree” and stuff like that.’

‘Wow.’

‘Yeah, that’s what I thought. Liv didn’t reply to the tweet about the wedding dress, but she did favourite it. Then, next time I looked, the tweet was gone. Vanished.’

Charlie nods. ‘Typical of Liv. She favourites to suck up, then DMs to say, “Actually, would you mind deleting that?” How did you find Natalie on Twitter with no surname to go on?’

‘I searched for “Natalie” and “McElwee” together – Nikhil’s wine company. There was something from a years-old McElwee Christmas party … Look, it doesn’t matter. What’s more interesting is: why did Liv want that tweet deleted?’

Charlie looks at me oddly. ‘She didn’t want anything online linking her and Natalie. I agree, it’s interesting – to me. I just can’t see why it matters to you.’

‘It’s my mystery too,’ I say. ‘You gave it to me in that godawful hotel in … wherever it was, and I’m keeping it until I’ve solved it.’

I can’t tell if Charlie’s impressed or annoyed.

‘So Natalie and Nikhil are getting married,’ she says, eyes darting back and forth as she processes the new information. ‘Liv and Gibbs must be their … best man and woman, witnesses, whatever. But so what? Why does that have to stay secret? Why has my sister never mentioned the name Natalie Burge in my presence?’