I bet you are still watering it as you did before?

Yup

If so it's a combination of the new cold conditions and too wet, let it
dry
off and if you can't put it somewhere warmer then keep it almost dry in
winter in future.

I assume if it's kept in a centrally heated house then it can be watered the
same all year round? Or do we still have to slow the watering down?

Also, as half of it looks dead is there any chance of saving it? I'm
guessing the brown bits aren't going to recover, so do we cut of the stalks
where the leaves are dead? Is there any knack to it or can we just hack
them off...?