Al Reynolds wrote:

What's the best way of stopping any nuisance trips
from the exterior lighting tripping the main house RCD?
I have a split-load CU and the three lighting circuits are
currently on separate MCBs on the non-RCD-protected
side of the CU.

Leave the circuit on the non RCD side but replace its MCB with a RCBO.
That way it has a RCD of its own and is hence separated from the main RCD.

--
Cheers,

John.

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