Update
First off, it is a protek board / rcbo's, not proteus as I first said. Don't know why I said proteus
Anyhow, tried the easy bits first, no loose neutrals etc
Disconnected rcbo earth tails, still trips basement power.
Last resort, which I knew would be pain, move rcbo basement 16a rcbo to far end of board
At first I thought fixed, no trip, went into overdrive on off on off basement lights, tried it 20 times or more, customer at side of me, no trip.
Right fixed!
Err no, packed tools up, turned basement lights off, friggin downstairs ring main tripped.
The actual basement lights never trip, always other rcbo.
As I said before, the basement lighting circuit is 3 fluros and one pir flood only, I can actually physically see and trace cables, no damage, all connections secure blah de blah..
Please remember, trip only happen when switching off lights, not on.
Headache, I'm going to fit a new board, Crabtree Wylex or MEM I asked the customer to get, (I didn't supply anything) see what happens...
Lost me this one