doesn't really look like either. It's not white spot, and I'm pretty sure it's not fungus. Her fins are quite badly torn up so it may be finrot, but I've noticed since my original post that the male was bullying her quite badly so I've separated her into an isolation/breeding trap in the tank. Temp in the tank is high due to the weather, about 28degrees, but the others seem fine. She's in there with her male friend, 4 platies and 6 khuli loaches.
Last night, I thought they were about to spawn as her ovopositer is very extended. I don't know if this would have anything to do with the male bullying her.
The only other thing worth mentioning is that I moved the pair of them from another tank the other day as every time they breed they lose all the fry to the clown loaches and tiger barbs in that tank! Possible that the stress of the move upset her too much.
Anyway, since I isolated her and added Melafix about 2 hours ago, she has perked up a little. I thought she was going to die earlier, but she looks a little more active now. I'll leave her separated overnight and keep treating with melafix and see how she is in the morning.
Thanks for the response. If you've got any other ideas, I'd be glas to hear them!!