Definitely a difference in the service you get depending on the engineer that comes out. Some have been great and replace stuff without any bother, others do as little as possible and then you have to get another call out because the job was not done right.
In the past I have done quite a few jobs on my now 16 year old boiler including replacing thermocouples, a heat exchanger and a thermostat. Found the info online to do most of the work. Reckon if you can work on a car then many of the boiler jobs would be reasonably straightforward, though obviously there are some safety issue to take into concern

Now if anybody on here can do Boiler Work then I reckon I could point them in the direction of a couple of jobs, and would be nice to know if I have problems with mine.