Tricky one....it might just be going through a winter "moult" I used to look after interior landscapes etc and the seasons are reversed....summer brings shade, air con etc where as winter brings heating and additional lighting.

Gradually increase the watering and see how it develops through the next couple of months

Did you check for any kind of infestation Hywel? Also are the leaves shriveling up before they drop off?

Just had a look on google Hywel and there seem to be several articles dealing with this - hope one of them can help.

Mine has a lot of yellowing leaves now,and a few shrivelling up,but it does this every year,so I put it down to them being the older leaves,so I regard it as normal,at this time of year..Do you have new ones appearing lower down the stems? Mine has got a lot on,so I don't worry about what I call natural dropping of the older ones..
When we were at our other house,it was kept in the lounge,which got warm,and it was fine,and now it is in the cold,and I think it's been better ..I would follow Sue's advice and check it on Googl.Sorry I can't be of further help,Hywel..and I hope you can solve the problem..

PS.Just checked Crassula Ovata on Google that Sue suggested,,and it's very helpful..especially,the first three websites,,I didn't need to look past those..One article also states what makes them come into flower..looks like it happens on maturity.or a period of cold..At last,I've found the answer for mine !!...so thank you Sue,I never thought to check it out before :o)

Happy to help - especially as I didn't know the answer myself!

It's taken me long enough too..For the past few years,I've posted a photo of mine in flower,and almost all the comments asked why mine,and not theirs,so it's helpful to everyone..:o)

Thanks for all your replies. I'll have a look later :o)