My brother has a system that has been covered by a BG service contract since new. So over 25 years. About 4 years ago they required a new boiler to be fitted and recommended a Potterton Puma. Which has gone wrong at least twice a year - and in a big way. The last repair a couple of months ago involved a new wiring harness as well as other parts.
However, the system is now broken again. And BG say it's blocked and needs a power flush - they've done their magnet test on the pipes and base it on that. Which of course isn't covered by the service contract. 500 quid if you please. But guaranteed for ever. And if the power flushing doesn't work they'll replace the pipework...