Thanks everyone.
Sorry should have said more about conditions. It's mounted, a little bit back from a west window (behind some other mounts). I water every day or every other day.
Temps range from 15C to 21C. I've not measured close to the plant, but that's what I get on a min/max thermometer in a nearby room and 15C is also the absolute min the stat should let it drop to even when the system is 'off' and 21C is about what we nudge the stat to when we are feeling cold.
I was starting to wonder about cold. I've tried reducing watering (not watering every day as I had been) and it has not seemed to make a difference.
... now of course as Camille has already heard, our heating has failed this morning
so the chances of me improving it's temps may not be possible just yet, in fact the house is going to be colder till at least Friday when the engineer is due to come out. I will try moving it to the living room though as that's the only room we can heat just now