rcd trip on earth faults. So you have a fault to earth somewhere.
Remove your cable from the boiler, check continuity between your cable wires L-N,N-E,L-E and you should get open circuit which means your cable and plug should be fine, any readings and your cables or plug has a problem. check continuity between the connection terminals on the boiler as the cable, if you get a reading between L-E OR N-E then there is def a fault on the boiler. (although this wont test all the boiler)
If you plug other items into the same plug socket and they dont trip the rcd then its not the fixed wiring.
Remove your cable from the boiler, check continuity between your cable wires L-N,N-E,L-E and you should get open circuit which means your cable and plug should be fine, any readings and your cables or plug has a problem. check continuity between the connection terminals on the boiler as the cable, if you get a reading between L-E OR N-E then there is def a fault on the boiler. (although this wont test all the boiler)
If you plug other items into the same plug socket and they dont trip the rcd then its not the fixed wiring.