either cut your losses now and replace it with something decent or (as above) sign up to BG maintainence contract – their engineers will at least know the issues with this shile of pite inside out because when they were first launched we were fitting thousands of them, until someone noticed that breakdowns had gone off the graph thanks entirely to the isar and its equally crap cousin the icos. at which point they were dropped overnight and BG have been throwing parts at them ever since
sorry to hear you've inherited one – i sometimes wonder how many people must think "the boilers playing up I know lets sell the house"