[Graham G3ZVT]

Every electrician knows this bodge "The borrowed neutral".

Said electrician renews the consumer unit and for the first time the house has RCDs, but one of them trips as soon as the landing light is switched on.

What has happened is the electrician who wired the house decades earlier, took the live from the downstairs lighting circuit and the neutral return to the upstairs circuit. It's so common it probably wasn't even regarded as a bodge.