I have noticed the gas valve drift, so I can see where you are coming from Dan. However this is an irritation for the servicing technician but has little impact on the customer. Similarly, the fact that the casings are a pain to get off and put back (about 6 different lengths and types of screw to lose and/or put back in the right place) is of no interest whatsoever to the customer.
Yes, there's lots that could be done to make the product more attractive for gas technicians, but I'd argue that reliability, in a boiler, is the number one.
With some of the more niche boilers such as ATAG or Intergas, you talk to a tiny technical team that know a small range of products inside out. The Worcester tech support team are sometimes a bit green, but there are lots of them and they answer the phone. Their admin people are efficient, polite, empowered; superb in my opinion, and that is what the customer sees.
It only this year that Worcester have the Which? game to themselves so, for now, their sample of existing boilers would not be unduly influenced.