my boss and father has 49 years experience as a gas engineer, worked for british gas for 35 years, 20 yrs as a training instructor and was regarded as british gas leading expert on combi boilers as it was he who set all the training up when they first started appearing on the market, in the days when bg controlled all the gas appliances on sale in the uk my father would get every new boiler sent to him at the training centre where he would strip it to bits, rebuild it, explore every possible fault and prepare the training manuals on it.
saying all that he hates combis with a vengeance, rates them as unreliable, difficult to fault diagnose and repair, give im an old fashioned conventional boiler with a pilot and no electronics anyday!
as said before, all combis need regular servicing and will breakdown now and again.
he as always regarded vaillants as the best combis on the market, closely followed by worcester-bosch.
he rates halsteads as good boilers but the parts are very expensive, ideals, vokeras, baxis, glow worms are decent boilers, not at all keen on ravenheat, chaffateaux, ariston or heatlines.
your getting a decent boiler for free so i wouldn't be too worried if i was you.
oh, and something i learnt last week, we serviced a boiler for a guy who had actually added breakdowns of the boiler onto his home insurance, only cost him an extra 24 quid a year, makes those monthy payment breakdown covers look a bit expensive, only cndition is he had to have a service once a yr for which we chage 40 quid.