OK - so heres some progress.
On Light B there's a RED/RED (one to loop and one to Live) and RED/BLACK (RED to loop and Black to N) - both earthed.
If I connect a standad bulb between L and N no joy. If I connect between L and E or L and Loop we have light.
Do you think the N return...
Thats my next step - I'll try connecting a simple bulb back onto light B and see what happens. If not I'll do as you say - thanks. I take it this form of wiring is unusual?
Help! I'm replacing a 1xdimmer and 2xbulb lights with 1xdimmer and 2xhalogen arrays. I got as far as replaing the bulb lights and every thing works ok. I went to replace the dimmer and lost track of what connected to where. Incoming to the dimmer switch are two Red/Reds (earthed). I can only get...