BG carry out a safety check that requires all tests ie burner pressure,safety devices,fluing ,ventilation, CO emissions, visual checks of the heat exchanger and conbustion chamber. If any of these are not correct then a full strip down service is carried out to rectify any problems. Also a full check of the system is carried out.
The BG contract states that any third party involvement such as a customers own contractor, are responsible for their own work they have carried out, regardless of what the problem was. BG will not repair any one elses cockups. If such a thing happened while BG engineer was working on the system then they would repair it on the terms of the contract. Very simple, cars are serviced and MOT's are carried out but they still break down, many soon after.