all thes forms about gas safe are if the gas fitter belives its safe. a flue is (id) the pipe is ok if the standing is no less that one mb from the meter to the boiler the meter box is not to standard some gas fitters class pipework as not to standard. i know that 2 not to standards are upgraded to at risk which means turning then boiler off. regarded the flue gases mixing he can say they were mixing without checking but has to then prove it as first thing you do when entering the house after getting a cup of tea is
VISUAL CHECK. im only learning gas and i would of done the same thing as the fitter but would of then offered to fix the problems
Take a deep breath; use some fullstops then it's back to the books I'm afraid. Almost everything in that post was wrong
Ps you were right about the tea though as long as the place is clean