Andrew Mawson wrote:
Been having lots of trips on my main RCD, mainly in the small hours of
the night. I've isolated each individual circuit, and checked with a
megger to earth for leakage and there is zilch. On the last couple of
nights I've manually tripped all the MCBs in the box before trying to
reset the RCD to eliminate external issues and it is very reluctant to
hold in, taking several attempts before it holds.

I suspect the RCD itself is knackered, and I've a replacement on order
(Wylex 30mA/100A) but is this a common failure mode for RCD's?

RCDs can fail like that. You will soon find out if yours has when you put
the new one in. Let us know what happens.

--
Adam