Hi folks, I think this thread is worth an update, two years on. TLDR: I believe it was a selenium deficiency in the ewe.
One of the lambs described above went on to do reasonably well. She was a little bit smaller than her friends, but not by much.
The other lamb never made it anywhere near slaughter weight by winter, and actually we had it PTS, rather than have it suffer on through the winter. An informal PM showed it had very little muscle, and that the muscles were quite pale in comparison to usual.
Fast forward:
She was empty last year and we earmarked her for mutton. However, that never happened, due to Covid. So, this year, we gave her an injection of vitamin E and Selenium four weeks before her due date. We then repeated that two days before she lambed, and she went on to give birth to three perfectly normal, healthy lambs.
Each of these lambs was given a further injection at birth, and are now doing well. I'll take the vet's advice as to whether more is needed or not.