I've been stalling on plastering the fireplace because of that wire. I feel I really should do something to fix it in place, at least. Even if it doesn't need protection, it's going to be easier to plaster if it's fixed. This afternoon I went downstairs to see what kind of rubbish we might have available. I found some bits of - I think Land Rover - head lining. Lightweight, insulating type stuff, easy to cut. I then sought out some staples that Ian salvaged from some pallets he broke up for firewood. They're flimsy things, but I can't find the longer panel pins that I'm sure we've got somewhere.
Anyway, I cut some strips of the stuff and managed to get two of them pinned to the wall. Only about a dozen staples were sacrificed in the process. I gave up on the third strip, but Ian pointed out that the wire should be fixed quite firmly at the bottom, because it's likely to get pulled. If it breaks up all the plaster, that would be quite annoying.
For the lower fixing, I smacked a tapered bit of firewood into a gap between the bricks and broke it off, then screwed an eyelet it. It was just the right size for the wire but, of course, far too small for the plug on the end of it. I had to pull apart the connection (on which Ian had fixed the insulation problem), then join it back together and re-do all that fiddly insulation.
Now I have no excuse not to do the plastering.