I'd repot it - it looks as if its not in a loam based compost (use John Innes No. 2 or 3) and it may be the compost is too wet. Never leave water standing in the tray beneath for longer than 30 minutes, empty it out, and do it again ten minutes later if more leaks out. It may also need a bigger pot by the look of the new growth coming up, but see what the rootball's like. These plants like well drained soil, indirect sunlight and temperatures no lower than 7 deg.C. Water freely in summer, but sparingly in winter - do it when the surface of the compost is just about dry to the touch, but don't leave it to dry out completely.

Regarding the compost it's in, I agree with Bamboo change the compost. Yours looks like it's in a compost that I bought some months ago from Lydl... Full of bits of woody twigs. I didn't like it and it seemed to hold a lot of water. I ended up throwing the rest of the bag on my border.

Thank you both for your advice. The compost is a mixture of fairly coarsely-sieved garden compost, soil and coarse grit. (My tomatoes are thriving in it in the greenhouse!) Maybe it's not free-draining enough but I've been careful not to over-water nor to leave it standing with water in the saucer.
Looking elsewhere led me to look for 'bugs' and with the aid of a magnifying glass I found something that might be a red spider mite though I'm no expert, relying purely on comparing it to descriptions and pictures. I've sprayed it so we'll see what happens next.