electrics tripping when water is being heated after filling bath?

when my Baxi Bermuda 401 lights up and is heating the water for the tank after a lot of water has been used (ie filling a bath) after a short period the RCD trips. The water pump on system near tank and the RCD have been replaced with new and fault still occuring.Can the issue be a fault within the boiler itself.
All electric circuits have been tested by a qualified electrician and seem to be fine.