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Anecdotal, but the guy who fixed our boiler was adamant that old boilers were much better quality than modern ones.
Define quality. Old boilers are far simpler in design because they weren't required / regulated to be mind bogglingly efficient and installable by a single human even if needed to be carried up four flights of stairs.
Do find it kinda unforgivable that appliance PCBs randomly die though.
I'm also considering air source but they all seemed a bit on the small side (eg around 16kW for a big one compared to 30kW for a large gas boiler). I do need to get some sort of sales person round though as my 40+ year old boiler isn't going to last forever.