Sounds like the flue has not been install properly and is sucking polluted air in, which will cause the syptoms you have when you connect the snorkel to the burner. Get the installer back to check it out. I imagine it wasn't commissioned correctly, as this would have been picked up.
On the IoW surely these cowboys must be well known?
Tony
You are right, being an island, you would have thought they wouldn't get work, do a bad job and the whole of the island knows, but there is still plenty of it going on.