Thank you for all your suggestions and good wishes for Polo. We did speak to the vet yesterday (she's absolutely wonderful) and we've given up on the cage. It was a big one but Polo obviously hated it. She's actually scraped her face rubbing it against the side.
Martin cleared out the spare room last night while I sat with an exhausted cat on my lap for an hour or so, and now Polo's shut in there. It will be home for at least a couple of weeks. She has to move as little as possible so the knee can heal. There's a mattress on the floor, and blankets, food, water etc, but nothing she can jump onto. Not that she can move much, poor poppet. She's mastering hopping today but yesterday she kept falling over. We're taking it in turns to keep her company - a good thing we both work from home! She's had a lot of tuna with painkiller mixed in, and I think she's sleeping.