Well said oz ,
@ShaunCorbs , matchless is simply asking advice , theres no need to bombarb him with does n donts as its just a question, he isnt asking how to do any gas work niether is he asking on open forum , hes simply asking advice which anyone can do , he should manage to ask a question without being made to feel like hes wrong in doing so, and also the building isnt up round the flue so as the picture is theres nothing wrong with builders starting while the flue is still in situ ,
@matchless your engineer will check manufacturers options of flueing also BS5440 ,
Guys dont get carried away with authority and help each other as much as poss, keep to forum rules n regs lol and everything will run smooth,