On further consideration I think a probable cause is this:
Earth leakage faults are overwhelming found in watery appliances.
the boiler, pipes, pump and motor valve are full of water.
the boiler, pump and motor valve also contain electrical parts.
If there is a small leak, water may drip out.
If the boiler, pipes and other parts are warm or hot, the water may evaporate away faster than it drips, thus they stay dry
but if the parts are cold (as may happen if the thermostat is turned down) the water may not evaporate away, and may accumulate sufficiently to cause electrical leakage.
If this is so, then the cure will be to find and rectify the leak.
You can look at the pump and motor valve yourself. Leaks will show as a whitish crust of limescale, or a brownish crust of chemicals, or green stains around copper pipes, or possibly stains on the floor or other parts. The leak might be at a joint that opens up when the parts are contracting, as would happen when the heating first goes off.
if you find marks and clean them off with a nylon scourer and cotton rag, you will easily see if new fresh marks appear.
You might be able to see marks around or under the boiler, but if inside it, you would need a boiler engineer to open it up. For some reason, some boiler manufacturers like to put the circuit board and connectors at the bottom of the boiler, so any leaks will drip on them.
Leaks from pipes concealed under the floor may show as brown marks on the ceiling below.
Does your boiler have a pressure gauge?