Thanks everyone for your kind words and PMs.
I'm sorry to say that this story doesn't have a happy ending. We found out last night that Lottie was killed on Railton Rd - probably the night she disappeared. We got a call from a lady who'd seen our posters, and had seen her body on the road on Monday.
It's very upsetting, but I'm trying to tell myself that at least we gave her a good home for the last eighteen months. She was a rescue cat, and had obviously had a tough life because she was never very confident around people. She was treated with kindness here, and obviously found some feline measure of happiness because she hardly ever left the house!
We had already decided to get one of the Albert kittens, who will be arriving in a few weeks, so Harvey won't be an only cat for long...
RIP Lottie.