I've made a little progress with this now. I took the door panel off. Lots of connections and things behind there but easy enough job. I unplugged everything and checked all the wiring for damage then refitted. Everything worked again, until the following day when it stopped again.
I did the same today and everything started to work. I went out to the shop but when returning to the car park it was all dead again.
Maybe worth noting that the box for the toad alarm is behind the door panel too. Could that be causing a problem?