I then asked another gas engineer to come round. He identified that the flue pipe wasn't sealed around the wall , hence CO gas was coming back inside. He sealed it, did some tests and gave the all clear.
He said he was and was recommended highly by a family friend.
The end of this story is I cannot install a plume extension kit as these are all plastic these days. I cannot find a metal one anywhere.
I have cut a part of the roof to allow for extra space around the flue pipe.
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