Thanks Aunt Sally and Nicky D for your help. I definitely think it is her crop - it seems to be getting bigger, but she's drinking such a lot of water that I think it is mostly water rather than food.
As for sour crop, I've read that if that were the cause she might be making jerking movements with her head/neck, as if she were trying to expel food? She's not doing this at all, but that said, it still seems one of the more likely causes.
We're giving her yoghurt and olive oil; we gave her another warm bath earlier in case the 'blockage' was at her back end, and we're keeping her warm.
We've had our girls for just over a year, and I put worming powder in their food every few months - the last was only last week. As for laying, it's a bit hard to tell because she's in with three other girls, but as she's smaller than the others I always assume hers are the little eggs! I'm reasonably sure, therefore, that she's been laying pretty much as per normal up until very recently (ie the last day or two).
We're keeping her in the house tonight, and I'll check her crop in the morning. If it's still full, I'll be straight on the phone to the nearest avian vet.
I love our little Rita and am really hoping she rallies around overnight.