Right, I have news... I feel like I'm one step closer to understanding what is going on with Holly, although it isn't great at the moment.
I gave all 3 pigs a one-off dose of Panacur. In my infinite wisdom (inbreeding) I decided to give her a day off the Daktarin oral gel, as I didn't want to mix the drugs up too much. Plus I had no idea if it was helping.
Anyway, her weight plummeted over the next 24 hours (even with some supplementary Critical Care), so I can only conclude that the problem is within her mouth and that she is eating less, even though she appears to be eating some of everything. She is therefore back on the Daktarin.
What has been confusing is that I obviously can't monitor her hay and dried food intake with the other two in the cage. I have seen her eating both, and she tucks into veggies. But I think the reality is that she is nibbling at hay rather than tucking in like she used to. My OH was watching her last night and said that she took a long time to finish a strand of hay, and then after it had all gone into her mouth she seemed to be chewing for a very long time. He had another look at her eating this morning and thought she looked like she was eating more normally.
I'm obviously also concerned in case it is her teeth.

I currently don't have a vet I can trust (I've tried out so many!

), so I'm terrified at the prospect of dental issues. There is no way she is going under an anaesthetic in this vulnerable state, but I really can't see that I could get to Vedra.