Soo let me get this straight:
From burner to loft in twinwall
From there in single wall flex liner to an old chimney and out?
The exposed flex liner in the loft space is the problem – this should not be exposed, onlywithin a chimney.
So if I read the regs correctly either:
a) find the brand of twin wall and use the same twin wall to make the connection to the chimney, then flex inside chimney and out the top. Or just take the twin wall straight up through the roof, and dismantle your chimney (no more leaks). This twin wall in the loft space technically needs a guard around it.
or possibly:
b) make a “chimney” to enclose the existing flue liner. Class A fire boards (hardiebacker) and fire sealant. I think the regulations will be vague on this one.