yes, the extended flue is surely the problem if it hadnt happened before, the boiler may be struggling to suck in enough air, to mix with the gas, gas biulds up while iar is comming in, when ratio is correct...boom.....
did you fit the flue yourself? are you corgi?
does the manufacturers instructions allow the distance and type of extention you did?
you messing with lives if you cant answer these above questions with a yes!
T, June 2008