i suspect a N-E fault on the boiler circuit.these faults can really upset RCDs. sometimes they trip when any load is applied, not always on the RCD protected circuit/s.sometimes they inhibit the RCD from tripping even on the test button. a thorough investigation of the boiler circuit is my call.
the engineer code should be the same as it was before. as long as it does not start with a 9, the codes can be reset. also, engineer tamper alarm reset can be disabled. tomorrow evening i'll try and root out the tech data to do this. i've forgotten it.
could be a large load being switched on fed off the same sub-station, causing a brief drop in supply voltage. e.g. motors etc. in a nearby workshop/garage.
OK. look at app.15 in BS7671. spur from ring final circuit. fuse down at socket that you're coming off, (13A), then a single 1.5 mm to heaters. D/P neon switch for each heater.