A clamp meter on the CUs earth wire can be useful (mA range).

A 30mA RCD will tend to trip at 23-25mA.
Several appliances with mains filters each leaking circa 1.5mA can
create a "background" leakage of 15mA making it very easy for another
appliance to push the total over the RCD trip threshold.

If the house does have a single RCD (TT supply or "whole house") then
conversion to a dual RCD CU would make life a lot easier when the
fault is found.