Many thanks to you all for your kind messages. It was a bit of a shock. My sister, who is a vet though not a bird specialist, was staying so she did a post mortem this morning. It turns out that although there was definitely sour crop that is not what killed her. She had a large growth and lots of lesions on her liver and heart. She also had an abdomen full of fluid. My sister thinks it might be avian leucosis but I took some pictures and will take them to my local chicken specialist vet to see what she says.