Are these faults related

About 6 months ago was asked to look at a fault heating (oil burner) not working and also no hot water. A quick flick with my over sensitive voltage stick showed supply was present at the  controller  the supply was was present but controller inoperative no indicator lights nothing. Not like the old days where a dry joint or burnt out resistor was detectable ,now sealed electronics so I had to get another one  ,this fixed the problem. However this week got a call heating off again and no hot water. This time there was no supply at the controller or thermostat , the fault was the supply a  40 AMP 30mA DOUBLE POLE RCD  230V ZR40 BS EN 61008. Whoever installed a new boiler about three years ago correctly used a small 2way consumer unit RCD with a 6amp MCB  along side it , one MCB spare. The RCD toggle had no switching action (not latch)even a bit of thumping no good. Now why would a RCD sitting in a consumer unit for three years never been touched switch mechanism  go faulty. Even when taken out and replaced I could not get the removed one it to latch. Something is common to these two separate faults. Thinking along the lines could switching on a boiler oil burner unit at the same time as the circulating pump and other hot tank controls cause a damaging spike , its very odd. Don't know if the RCD has a electronic latch or mechanical latch , but I will go back and get it if not dumped and cut the dammed thing open to look.

Thanks for reading this 

jcm