I didn't say it couldn't be done by a diy er, people seem to be under the impression it's an easy diy task. It's isn't, I not saying it's rocket science but like a lot of things there's far more involved than would first appear. A for instance is the checking of a flue how many diy ers know how to pressure test a flue rather than just carry out a smoke bomb test ?
It has to be signed off....I don't sign anything off if I haven't done the work ......why would I put my neck in the noose for anyone else ?
Regarding your comment on professionals and poor work your absolutely right unfortunately there's no register in any trade regarding unsupervised conscientiousness! !
Worcester bosch boiler oh dear oh dear.......gas safe inspector quote " worceter bosch are living on a reputation they got 20 years ago."
Lost a quote because the customer wanted a Worcester bosch boiler fitting and I refused to fit one, payed £800 more than my quote for fitting a superb boiler !
Google reviews for Worcester bosch .......
I agree with your first two paragraphs.
WB are a good brand, recommended by Which, for what that is worth. The engineer who last serviced my boiler said they were very reliable, he services loads of them, reckoned I'd have issues with hard water deposits at some stage as I have very hard water. I had a 3 year warranty from a non accredited installer, the accredited one wanted to charge too much, probably to cover the accreditation fees (a bit of a con I suspect).
When I was choosing a boiler, I found that gas engineers all said totally different things. One constant statement is that modern boilers are more complex and do not last as long as older ones. My original boiler was an old floor mounted job, ugly, inefficient, at least 20 years old and working.