baxi combi instant 105e?

i have been told by a plumber that the reason my boiler keeps tripping and wont stay on is because the differential valve is leaking onto the electric bit next to it, he is unable to mend it for me because he has gone on holiday but he told my son how to fit a new valve, i cannot afford to get anyone else in at the moment but i am a bit worried, is this quite a simple job and would it be safe to let my son try?
i do have the maintenance manual that came with the boiler and the correct part numbers, please help.