I find that at the mo guarenttees arent worth the paper they are printed on. all manufacturers are tesing / checking the installaion and service records of boilers before starting work under warranty. no boiler form the better manufacturers will devlop a fault in the first 5 years if installed serviced and maintained ( certainly with hand on heart i have had to repair a boiler that i have installed)
Bg do a great job but will sell you everything they can anything else is just cost reduction
buy what you can afford all installaers have thier favorite brands and prefer one over an other
i base my choices on those i find easiest and quickest to install/ repair and those that have the best manufacturer back up and those whose parts are cheapest.
from that on balance 98% of the stuff i fit is worcester bosch for those on a very strict budget i do install a potterton heatmax.
replacement WB start at 1250 inc to 1850 max replacement pottertons start at 1000
i have just completed a bungalow total replcement boiler pipewrok 6 rad a towel rail digital wireless controls for 2575. ( 5 yr warranty on boiler 2 yr on all other work)