OP
Hawkmoon
Thanks, guys, for your interesting replies.
1. RCBO required because cables to new wall lamps buried less than 50mm below the surface.
2. Testing - IR testing between circuits is useless if there is no light bulb in the landing light and it's not switched on! I fkd up!
A good indication is to check if there's a live strap across both switches of the usual 2 gang downstairs switch point. I didn't - in too much of a hurry!
3. Solutions - I prefer to feed an upstairs live down to the downstairs switch. If the upstairs pendants are all looped it can get messy separating out the upstairs neutrals and feeding in a downstairs neutral inside the pendant, apart from the safety issue of 2 circuits within one pendant. Inside the switch it is easy to leave an isolation warning note.
4. Not happy. I shall go back to the customer, either to persuade him to have the remedial work done or to put both ccts on the same RCBO as a Risk Assessment allows.
Cheers
Pete
2. Testing for b/neutral - forget all that palava, just turn on landing light and use clamp ammeter at the CU to determine which neutral is carrying return current - quick/easy/failsafe (takes about 1 minute if you're slow)
3. Better to take the neutral upstairs, that way you are keeping the landing on separate circuit from the rest of upstairs - why not just jb (separate) the upstairs circuit outside the pendant? (assuming in loft)