Now, having concluded they were dead, that area hasn't been weeded, so that's probably not doing it any favours. It's halfway covered in vetch, for a start. But even if I weed it thoroughly and carefully, I'm not sure what to do to actually encourage the plant. More compost? Dig it up and move it somewhere else?
Some initial research seems to indicate that compost is indeed the way to go, but also argues for a well-drained sandy loam, rather than our cold clay. So I'm reckoning that if I want to keep this one, I'll look at clearing it up carefully, leaving the fern there over the winter, and providing it with a good dollop of compost in the autumn. But it does seem like a small raised bed elsewhere with some more suitable soil brought in might be a better solution - it'll warm sooner in the spring, drain better, and be somewhat easier to keep weed free. I do have a bunch of beams left from the attic, so I could build a nice high bed.