£20 for a Certificate, that guy is letting the side down.

It cost on average about £1000 every 5 years for Gas ACS training and tested after, thats just to work on domestic appliances. treble that if you want to work on industrial.
Plus £200 a year for CORGI registration. Plus everything else associated in running a business. That money has to be recouped some how.
I would never recommend anyone work on Gas or the Gas part on a boiler or fire, if not trained. I've seen the aftermath of electricians, joiners and even plumbers (who should know better)when they have worked on gas with only a little knowledge.
If you get a leak on a water pipe the worst that can happen is someting gets wet. Get a gas pipe leak in a confine space and you have a potential bomb.

I personally dont think changing a programmer like for like, is a big problem. Having said that is the problem futher up the control line and you blow the new programer. They wont take it back then.

I'd draw the line at diy gas fire fitting though.

Has it been tested with a calibrated carbon monoxide tester, the fumes can kill as well as gas.