Views wanted...

[Highresdis]

Hi guys.
I wonder if I can inquire of your collective knowledge on this one. A colleague passed me a job where the customer had a couple of faults.
One was that kitchen lights would trip the RCD when switched on. The installation is on a 16th edition split board, and none of the lighting circuits are on the RCD.
Interestingly, the trip occurs only first thing in the morning, after which the lights can be switched on and off with no tripping.
I'm scratching my head. I tested the whole house, and found no other clues. What do you think?
Thanks...

 

[Rockingit]

Heating circuit has stolen a neutral?

 

[Highresdis]

Good possibility. I can't think where it would be; the immersion and 2 storage heaters are all on individual radials, but I'll definitely search for that when I go back.