Thanks Kate. The advice is much appreciated and makes me realise there's no hard-and-fast rule.
Fate intervened, as it happens. I had to put Molly back inside and hoped that when all 12 of her piglets went off to new homes, she'd get used to the idea and go out. Then one buyer dropped out and two of the male weaners picked up minor injuries so they are now back in with mum. They aren't attempting to suckle and everyone seems quite happy with the arrangement.
It means Kim is on her own for now, but I'm hoping Molly will go back with her in time, along with two gilts from Kim's litter. Fingers crossed.
And I totally agree about intensive farming. It may be clean as a whistle, but that doesn't make it any less cruel.