Bear in mind Baxi does not seem to be able to ensure its "Approved" installer network does not include a few bad apples(came across my second Baxi approved cowboy installer recently), but Baxi certainly does pay out on its warranty if you use an "Approved" installer. Though, ideally, find a good one.
Of these two Baxi-Approved cowboy installs my customers have come across, one had a new heat exchanger fitted under warranty after original cowboy left system water so dirty that it got blocked. The other (fitted with apparently no thermostat, although the Benchmark certificate says there is, and installed with the condensate pipe terminated directly onto the driveway rather than to lime filled soakaway or to foul drain) also had £1000-odd of parts replaced under warranty.
Whether it's ethical for the parts to be so expensive in the first place is another matter, and I'm still undecided as to whether I will ever willingly replace my 1988 boiler (small and reasonably insulated/airtight house, so bills are quite low anyway), though in principle, I'm all for greater efficiency.