Longish story but I wonder if anyone can suggest any reasons/ ways to prevent in future...
I noticed yesterday that she wasn't keen to jump over the edge of her run to get out into the larger area, but once I had helped her out she seemed fine - eating very enthusiastically. Then last night I spotted her sitting still in middle of lawn, no effort to move when I picked her up. She still ate a bit of food when I put it in front of her so I just popped her into bed hoping she'd feel better by today.
This morning it was clear she wasn't at all well - not eating or moving. I put her in a box near the house and kept trying to persuade her to eat or drink, but no luck. I couldn't feel any egg-bound bulge or impacted-crop bulge - she did feel very thin though. I managed to put a bit of water into her beak with a syringe, but sadly she died at lunch time.
I think it's possible she had worms as she felt so thin - we have used Vermex but I will treat the others with Flubenvet now incase it was that. She was an ex-batt, we'd had her for a year, so I suppose she might have just died of old age.
Don't know if I could have done anything else?