Boiler Electric problems.

[Barry Conlons wig]

If the RCD is tripping in the mains board and the circuit breaker isn't tripping it is a neutral to earth fault on the boiler
It could be anything so you will require a heating engineer to look at the boiler and he will have to try and find the earth fault which will be a process of ilimination
My bet is its the PCB (printed circuit board)

I'd bet the same as this, however if you told us the boiler make it could make it easier to try and identify the problem