Your best bet with your needs, ie wanting a first time fix is to go with a boiler who's manufacturer uses mostly directly employed engineers. In which case I'd be looking at the big players such as Glowworm, Vaillant, Worcester, Baxi and Potterton. I used to be an engineer for Vaillant/Glowworm and first time fix was usually around 90-95% on all boilers and prob around 98% for in warranty stuff. I expect its the same for the others mentioned. Very few other companies use directly employed labour asfaik and therefore less likely to carry spares with them. I have heard terrible stories about Ferrolli backup, something you'll need cos the boilers are absolute garbage.