Parts for your Dad's boiler seem to be readily available, so if the gas guy claims otherwise and that he's trying sell your Dad a new boiler, find someone else. 
Gas Valve
PCB
Yes, plenty of genuine parts, think theres enough to build an entire one. Thats the thing, he didnt quote, for anything. Just walked. He doesnt want any job for whatever reason, hence researching, and he's found another company.
Its more at 30 years old, do you sink £500ish, or cut losses.
Other factors being his age, and what the future brings, do you buy normal now, or have heat pump forced in a few years.
This one is basic, suits him, heating and or water on twice a day, one button to boost on or off if required, no messing about with awkward programmers etc
