some googling seems to say here are fittings to which you just seal the membrane further out from the flue with fire-mate or similar and keep insulation back a safe distance. easy as that? cement board?
etc etc
thanks

Be very careful. Clearances to combustible material mean it must not be within the specified distance and no amount of heat proof material will make it compliant.

I also know of a fire caused by a flue that passed through a wall, all the clearances were as they should have been ie no combustible material within a certain distance, but because the building was very air tight (almost Passivhaus) the gap was very well sealed. This caused heat to build up due to lack of air movement and the place caught light and nearly burned down.

thanks bear.
the gap was very well sealed meaningaround the fitting?

the chimney cavity around the flue pipe needs to have a specified amount of air flow seemingly, in france anyway, where i am.