Hi Danny,
bet the neutral is on the RCD side!
look at the 2 neutral bars and count cables on each and compare with number of lives on each half of board. IE 5 lives on RCD half of board should have 5 Neutrals on RCD side of Neutral bar. RCD is looking for balance so if one circuit is on wrong half the return current will bypass it & it will do it's job,
Regards SPARX
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