Yeah but..... When one Googles a problem you will find that the 'hits' have no respect for age!
Thus this is for anyone currently seeking answers.
I would guess that the original guys have fixed their problem - or invested in hot water bottles
beware! If it helps.....
15 Apr 2016
By andysceptic
Berkshire
" For a couole of months the RCD intermittently would trip. Got so frequent that RCD would trip instantly one circuit was activated, then only when central heating turned on. Identified culprit - the new Grundfos pump fitted in...