What does service properly look like they check for gas leaks check pressure and turn on heating. They haven't bleed the radiators in the whole 6 weeks they been working on it but they have emptied the water tank twice. Someone was calling today but didn't show so called and apparently they are coming out tomorrow. Different engineers which seem to not be talking to each other or looking at the previous issues. I have an autistic son age 16 and this is in his bedroom and he texts me to ask if it's going to blow up when he's asleep.
A proper service is what the boiler manual dictates. In my book that is a minimum requirement.
On the boiler one would be checking the condensate trap, carrying out flue gas analysis, might even check gas consumption if a flag was raised by the enduser and also check the red vessel for charge pressure. Gas leak- not relevant unless it again has been mentioned. Oh, check system water though I rarely do that
Radiator bleeding is an old wives tale. Know an old lady who habitually bled the radiators every week until she dropped the bleed pin and never attempted to bleed the radiators again. If the radiators need bleeding then there has to be an issue that needs addressing. Bathroom towel rails collect air for fun but I suspect it must be materials used that breaks down the water but I could be wrong
WRT cylinder. Check tundish and that white vessel for pressure. The manual may ask for other checks.
Boiler might only blow up if every safety device has been over ridden- it will in most cases fail long before it blows up.
Your cylinder on the other hand could just do that but even then you would need to push it hard on every safety scenario and then some and it might even then refuse to blow up.
In a nutshell. There are first, second, and third level of safety built in to stop catastrophic failure
If above service was done, then you would not be having issues you do.
You cannot pick the gas engineer, those folk who can are in a better position as when they call their engineer, the engineer is familiar with the setup and the history