Just to add and remove uncertainty of earth leakages.
Isolate and test supply downstream of meter, unplug all appliances, ensure that all lighting ccts are off, dimmers switches off.
Insulation test (Megger) the installation as a whole or circuit by circuit at the time. readings should be in the Mega ohm range for the wiring concerned.
Any lowish readings less than 1 Mega ohm on a circuit should be further tracked down to the cause.
Also label all sockets afterwards with a dymo machine and charge customer as an extra service if they want it.
Just my $0.02 worth on this topic.
