Well I managed to fix my daughter's bathroom lights but what a nightmare!
If you remember there are 10 lights in the bathroom with 4 switches.
The switch has 2 input supply cables with the lives connected to the switch commons, neutrals connected together. Live and neutrals plus earth go to each set of lights. No power on either of the input cables although the neutral on one was connected I discovered.
Checked all the adjacent ceiling roses, no loose wires.
I jury rigged a supply cable from a socket to test the lights, this tripped the main earth leakage RCD until I disconnected the input cables.
Lifted up the floor boards in the room above. The bathroom has a false ceiling fitted below the original 1920s ceiling. The wires from the switch disappear into the false ceiling and there no way of seeing where they go. [one of those cameras surgeons stick inside you would have been useful]
I can only assume there is a junction box somewhere in the false ceiling.
Anyway in the end I ran a new cable from the lighting circuit to the switch and safely terminated the original input cables. The lights now work.
Next time she can get a proper man in!