[bodgit]

Semi sorted. Electrician just called now. All the switches in the box lit up his gadget so have power. He thinks it a probably a loose neutral somewhere in the circuit. too late for tonight but will fix it tomorrow.

Ok he came back and found that it was a loose connection sort of. The electrician who wired the house when it was built 22 years ago was crap. The wire that was causing the problem had only had about 2mm of the sheath removed so it wasn't actually screwed into place, it was just resting against the connection.
I wondered why for he last 15 years every time somebody was heavy footed upstairs the lights would flicker, the wire was jumping on and off its connection. So if you have the same problem turn your power off and check all the light wires are actually screwed in.