The going rate for a professional gas service engineer is more than your engineer is charging, you had done your home work and you were able to establish this, DGOBS warned you:
" I have to say I seriously doubt anyone registered could afford to service and appliance for €50, seeing as the vat would be 13.5% the gas cert would be roughly €5, then overheads plus running costs, then subtract the tax, you'd be left with nothing!
you get what you pay for".
I couldn't afford to do business at €50 especially with the extra charges as a RGII engineer, my point is competent and registered engineers come at a price and we are only as good as our last job, so that tends to be the motivation to repair in one visit, you wrote that people are at the mercy of trades men, i disagree, you do your homework, find out what's a high/medium/low price and make a decision, you chose cheap,which is fine for a service but faultfinding takes a lot of skill, also there's lots of engineers out there who are honest and don't rip people off.
My advise to you which is happily free

, would be to contact Glowworm: [broken link removed]
you can do this yourself and give them the symptoms and hopefully they can advise you or get your RGII engineer??? to ring them when he's on site, also if you don't have the boiler manual, see if you can download one, try and get as much information on the boiler as you can as the problem could be anyone of numerous components that are responsible for ignition, so either replace them all or find a boiler technician who understands boiler faultfinding, we are a bit different to service engineers, Gary