I've got a can of lithium grease that's begging to be used, so I'll be doing that this weekend.
Probably just did something very stupid - walked past my local petrol station and they had diesel, so jumped in the car and drove up there, even though there's only one bolt in the rear shock. It started coming loose after about five minutes and was making a hell of a noise, but I have diesel now at least. It wasn't torqued, so that's probably the reason it undid itself, and it does do up tight, so I think I just made it and haven't damaged the threads.
Of course, I found that my tap and die set wasn't at the girlfriend's house, but was at my house the whole time, so I could have cleaned the thread earlier in the week. I'm going to sort that tomorrow, but today I cleaned up the threads on the bolt that has damaged threads so I can get a feel for how the tool works. I'm hoping I can get the tool in OK with the space I've got in the wheel arch, but there's always an adjustable spanner I suppose if I haven't.