Thanks guys. I’m going to have a look this weekend. I’ve had two central heating companies out to service the boiler and neither of them has suggested these as causes. It’s an oil-fired boiler, and I’ve noticed some carbon(?) deposit along the plate at the top, which presumably is a slight exhaust leak. Maybe that’s all the problem is, or am I being too simplistic? Anyway, I’m going to turn it all off and take that top cover off and see what I can see. Any ‘dos or don’ts’? Any easy way of checking for leaks without killing myself?