Thank you Gill - her and her mum are so precious to me.
I did try to keep her in my room last night, but after 2 hours of constant meowing I carried her outside. She went straight to her normal patch and did a wee. When I bought her back in to my room, she tried desperately to get into the hallway. I tapped the litter tray and tried to persuade her that that's where she needs to go, but she just wanted to go out into the hall. She did eventually go in the tray, but the fact she waited before she went was proof to me that she can hold it, but for some unknown reason she really wants to wee in the hall. We thoroughly cleaned the carpet there and her litter tray is always kept clean, so I don't understand. It seems she would prefer to go outside, wee there, and then do the rest in the hallway. She's not pleased at all at having to be kept in the room and using the tray. Yesterday she did actually wee on my bed, either when she was asleep or just after she'd woken up. She seemed quite shocked by it anyway. It seems to me that most of the time she can hold it, but the odd time she can't. This is all so confusing! I wish she could talk!
EDIT: I've just thought that if it is cystitis she's associating the tray with pain and that's what is making her reluctant to go there, but wouldn't she just do it somewhere else in the room?