RCBO tripping 2 way circuit

[Glenparker95]

Since DB upgrade 2 way top stair light is tripping rcbo, it has a separate N&E to the feed supplying it im assuming this is the reason?

 

[westward10]

Almost certainly a crossed neutral with another lighting circuit. It was common in the 70s for the landing light to "nick" a neutral from the upstairs lighting circuit when the landing light is supplied from the downstairs lighting circuit.
Your electrician should have checked this prior to the change and must have been aware after.

 

[DPG]

Did you get an installation certificate with test results?