Chris, ITL,
Yes, just googled the prolapse pics and it definitely looks like that. Mine is a Black Rock too, perhaps they are prone to it.
She is already isolated and I have taken away the layers pellets and corn, just given her some scraps. I can't catch her on my own so the medical bit will need to wait until later on when OH gets back.
I've just been to see her and although she doesn't like being shut in on her own she seems to be fairly ok.
Thanks for all the advice
Sally