Thanks Will, I'll try to give you the info I missed out last time!
Watering - about every three weeks because this gauge keeps telling me the soil is wet. Also not sure if you can advise but I've no idea how much water I should be giving it... soaking the soil, or just a few cupfuls.
I haven't repotted it, and have included a picture of the pot below. It has drain holes and then stones sitting between the plastic pot and the stone outer pot.
The photo also shows the position of the plant. It's a very light room, but it's not right in front of a window, I don't have room to keep it there as it's quite big. As I said though, this is only a recent problem and I'd have thought that if it were a light issue, it would have been a consistent problem throughout it's life....
Anyway, hoping this extra info will help you.