Belated update
The ewe has lambed and although it was a tricky birth there's been no sign of a prolapse since the birth. Whew!
I did manage to get a harness when I had to take a different ewe to the vet for emergency caeserian (this one had ringwomb), and I found (at last) a youtube video that showed clearly how to put it on, so now I know! It worked well.
The reason getting a harness was a problem and why the vet almost came out here was that I'm recovering from the virus and, until the emergency trip to the vet, haven't been anywhere myself. Neighbours have been fantastic and have been helping as much as they can but my farming neighbours are in strict isolation due to needing to protect older family members.