I'd like to thank the posters on this forum for the advice posted here, which I turned to recently dealing with an impacted/sour crop, which I was able to diagnose/cure(fingers crossed). Hen had been behaving withdrawn for a little while - not racing out to feed in the morning, but otherwise looked fine, no obvious signs of distress. One morning wings were droopy, so I knew something wasn't right. Crop was full and hard. Into the isolation tub! she was pooing a bit, but not enough to clear the crop. Started off with a couple of teaspoons of olive oil, massaging the crop until it was pliable. Forced some sugar water down there too to get things moving. Further massaging resulted in a very foul burp - sour crop. She was still eating, so I provided a fairly fluid mash of yogurt and pre-soaked pellets, plus some fresh apple. I continued to massage the crop for the next few days. After two days I let her outside to judge behaviour, but she still wasn't normal. Continued the yogurt and crop massages, but supplemented with some dirt in a separate bowl. She was looking a lot better (more attentive/wings up), passing things fairly well, but still had stuff in the crop. I left her off feed for the better part of a day. Gave her some more yogurt/mash and a bit more oil. The next day I took her out again (before everyone else was let out) to see how she was behaving. I'm happy to report that she is now back in the flock and behaving normally again.