hi rosie, i'm a trainee heating engineer, i work with my father who has 48 yrs experience, 35 of those with british gas including 20 as a training instructor and he was regarded as bg's main combi boiler expert.
as far as bg's service contract goes, as with any other gas companies service contracts, they aren't prepared to take a gamble, they are edging their bets so that they only take on boilers that they think aren't going to cause them problems or cost them money. we recently serviced a boiler for a customer and bg had cancelled their service contract just because the boiler was 10 yrs old.
bg also have a habit of telling everyone that their boilers are obsolete or beyond repair and then trying to sell them a new one, (the engineers get commission for every new boiler sold) be very wary of this, if you find an older very skilled gasman they can often keep the old appliances running for years to come..
bg arent the same company as they were 20 years ago. the govt decided to sell them off and break their monopoly, they used to have the best trained engineers bar none. since the sell off they laid off a third of the workforce, closed all the depots and most of their training centres and stopped taking on apprentices and pulled out of most work except central heating. now they have decided to try and get back into the repairs, installation and servicing market of fires, cookers, warm air etc so have had to take on a load of new engineers who haven't had the training that the old style engineers did.