hi BingoBongo i attemped to contact you by phone 2 days aggo my fitter was with me at the time you seem to have the knowledge if you would be as kind to speek to him you can assure your self of what you need to know before you tell him any thing, would you be willing to speak to himthe fault could be tested/cured in 5 minutes but as this involves breaking the combustion seal nobody will risk telling you what or how to fix it incase your engineer doesnt exist and you are diy.If your children are indeed very important to you,then i advise using some of that gas and liquid money to employ a different rgi or maybe put him through a training course
Kind regards Neil