Looked at images on google and realised it was definitely a prolapse. Took her to the vets and he said that what appeared to be labour (she was due yesterday) was in fact her expelling the prolapse. He pushed it back in, gave her an epidural so that she wouldn't try to expel it again for a bit and put a stitch in, some pen & strep and some anti inflamatory.
From the internal examination he doesn't think she will lamb for a day or two but will then need an assisted lambing. We have decided for first timers we will get the vet to assist. He has told us what to watch out for and then to ring and take her in again.
I think I must be jinxed. My first chicken had a prolapse and now my first ewe to lamb
Good news is that she seems as bright as a button and is eating well.