Thank you all for your kind words. It has really helped.
We ended up asking the vet to do a basic PM on her, as when my friendly neighbour came over to help me get her in from the field, he agreed with what some of you had said, and that a young, non pregnant sheep shouldn't be getting cast, and that given the weather recently, and that we had had to dose twice for fluke (not a known problem in this area, but neighbour had had some resistance, and I have been doing FEC for the horses anyway, so added the sheep, and found dose 1 hadn't cleared them) that we should check for other causes. Laying as she was, with her head in water and more on her back, than side, had led me to believe she was cast, but the vet said there was nothing in her lungs or airways, but that her heart was enlarged on one side, which he said could have been due to a minor heart defect. We are due to have our 3 pregnant ones ( different field ) scanned in a week, so the vet has agreed to have a look at the lambs twin sister, and their Mum, while he is here, and see if there is anything to see on scan for their hearts. He has said that the scan is only likely to pick up enlargement, or gross issues, rather than subtle changes with valves, which is a far more specialist role than he is up for. We had already decided not to breed the Mum again, but would add the twin to the lawnmower group if there is any suggestion of heart issues with her too.
They have shelter in the field, but are not often in there, although I do put hay in there for them, and they had been in there eating hay the few days before, as we had quite a layer of snow for about 72h. The mineral lick is there, and the vet suggested adding an energy bucket as well, so I have done that now. They do get sheep pellets daily, more as a routine, and to get them into the barn for checking. Nobody seems to have dropped weight ( or got too fat ) but the 4 Mums are 7 years + now, and this is the lambs first winter, so I wanted to make sure I could keep a close eye on body score.
A steep learning curve indeed, even with excellent advice from neighbours, the internet, books and the vet. Thank you all again