Neither of us had. Tilly was delighted and wheeled herself along beside Charlie towards the big wheel, the two of them chatting like they’d known each other for years. Charlie gesticulated as she talked, her face animated. She looked like a movie star, the girl next door in an old American film, and I was struck by two emotions, one immediately following the other: joy, that she was with me; and fear, that at any moment she might disappear like she did after our first night out. I told myself to get a grip. Relax, enjoy it. She seemed to like me a lot. The way she looked at me reflected back the way I looked at her. And if she didn’t care about me, didn’t want to give this budding relationship the chance to bloom, she wouldn’t be here now, taking my sister out, would she?

The London Eye was even better than I’d hoped, the city stretched out before us, proud and ancient and alive. Charlie pointed out her favourite buildings and Tilly recounted the time she and ‘a load of other wheelchair kids’ were taken to Buckingham Palace to meet the Queen. Charlie had a related story, about how the Queen had come to their school in Leeds and they’d all stood outside waving flags, hoping she’d brought her corgis with her.

‘I’m not a big fan of the royals now, though,’ she said.

‘Oh, are you a republican?’ Tilly asked.

Charlie waved a hand. ‘Actually, let’s not spoil the day with politics.’

I knew already, from watching the news with her, that anything Charlie saw as injustice made her angry. I had listened to her rage against some new policy the government had brought in, the so-called bedroom tax, and halfway through her diatribe I’d had to calm her down, pointing out that I wasn’t the prime minister and couldn’t do much about it. I liked the fact that she cared so much, though. It was another sign that she was a passionate person.

After the London Eye, we went on to Trafalgar Square and looked round the National Portrait Gallery.

‘Charlie’s an artist,’ I pointed out to Tilly.

‘An aspiring artist,’ Charlie said.

‘I really want to see some of your work,’ I said.

‘You will. Maybe you can pose for me.’

I was taken aback and Tilly laughed. ‘If you get Andrew to pose for you naked, please don’t ever show me the picture.’

That set them off again and led on to a conversation about penises that got ruder and ruder as we walked around the gallery, the two of them giggling like schoolgirls and pointing at portraits of historical figures and rock stars.

‘Six inches, I reckon.’

‘I’d say he’s got a nine-incher.’

‘A disappointment.’