After some research I did come across impacted crops. I've now fully emptied the crop - the contents were smelly but not revolting - so i've ruled out sour crop. Without the fluid the crop is now very small, when I massage it I can feel some gritty bits inside but nothing that feel's like a ball of food. I do feed her small amounts of oyster shell as she only free ranges 1-2 hrs a day.
I read that white maggots were good for impacted crops, I was planning on getting her some probiotic yoghurt too. However she has no appetite. About 10 minutes after emptying the fluid from her crop she has drank about 100ml of water. I don't want to keep her off the water and let her become dehydrated, but i dont want to put her through the crop emptying again!