Sitting watching t.v. tonight. I happened to notice that the burner on our boiler seemed to have been running a long time when suddenly there was a bang and it cut out. I'm a bit reluctant to attempt to start it again. Any ideas as to what could be the cause. Don't want to spend a lot as we're changing to a combi in a couple of months. Existing is a Myson Housewarmer back boiler.