Apart from the fact you live in the UK, there's no clue as to quite where in the UK you live. However, most of the UK has been very cold during the last few weeks - if your heating is off at night, it's possible the plant is exposed to lower temperatures than it likes, and that's why its dropping leaves. This assumes you've inspected the plant thoroughly, all over, for signs of infestation or disease.

I think it might be too warm - rather than cold. My lemon tree is in an unheated greenhouse - with a blanket over it when v. cold. It has survived many cold winters that way. If you can't move it i would suggest letting it dry out, then maybe stand on a tray (or next to) of wet pebbles. The dryness in the atmosphere can sometimes trigger leaf loss. But do check for bugs - especially red spider mite - under the leaves.

Jane, it sounds like it's warm enough and moist enough...so if you haven't recently moved it in from cold and shocked it...I think I'd be taking a close look to see if there are any signs of red spider mite. Citrus plants seem to be one of this horrid pest's favourite homes! Tell tale sign is a fine spidery web all over the leaves and yellow mottling on the leaves. Other possbility could be vine weevil larvae. To check for them you'll need to take it out of the pot and have a look for smallish white grubs. They are easy to spot. if you find them, pick them out and then treat it with a nematode treatment available in GCs. Also, I recently realised that citrus need feeding all year round with specialist citrus feed ...winter one in winter and summer one in summer! every 10 days. I do hope it gets better for you...mine is just beginning to flower in the conservatory and the perfume is wonderful!