A few months ago - during winter, so it's possible that I over-watered it as it's a very different (colder and darker) climate from what I've kept the plant in before.
If it's root rot, I should just repot it and clip off any bad looking roots in the process?
The fact that they came back looking bad again makes me wonder if this might be the case. It didn't use to respond to under-watering like this in the past (older leaves would die), and I don't think I have been over-watering it lately either.
It's been repotted fairly recently, and the problems seem to have started shortly after really. Maybe it needs to get used to the new pot, but all the new leaves start looking bad after a little while at this point, and the part of the plant that's already grown seems to be gradually getting marks and looking worse.
Thanks for the advice on watering - I'll be more hesitant going forwards! Could very well be that I have been over-watering it, in the past I got the impression that I could do nothing wrong with it.