Electrical fault on my Danesmoor oil boiler?

I have a problem which nobody seems to know the answer to. My boiler keeps tripping my residual circuit to earth breaker. The electrician has tested all my leads and can find no fault. The temporary cure was to change the oil pump as with the lead to the Solenoid disconnected it did not trip. This worked for about an hour after the engineer left and then failed. Both the engineer and the electrician returned. The electrician found that the lead to the pump and fan motor had been nipped and taped the lead up. After this the boiler went for about 3 days then failed again by tripping the residual earth breaker. Now it will go for about 10 minutes at a time once it cools before it trips. It will still not trip the breaker when the solenoid lead is disconnected from the pump. ANY IDEAS