GRP and twin-walled chimney

[SteveB64]

Hi DIYNotters, I have a flat GRP roof installed by a builder, through the roof is a twin-walled chimney from a wood burner. Roof was installed 3 years ago. The builder (reputable) replaced a flat felt roof including the boarding and ran the GRP up to the twin wall flue without flashing. We used the wood burner for two years without issue.
Then... we remodeled the room and decided to move the burner; hearth was rebuilt as per regs and the new angles for the flue pipe are also within regs, confirmed by two separate stove installers, and here is the question. One installer said nothing about the twin wall being GRP'd without flashing, the other said we needed to cut a hole in the roof, with 50mm clearance and install flashing.
Whats the deal? The installers that said nothing were from a reputable company - should I get a third opinion?
TIA

 

[bobasd]

the clearance should be about 60mm, and some kind of guard fitted to hold the flue centred.
you should get a HETAS in for a view.
Your insurance should be notified, and they might want a HETAS Cert of Installation?
Your BCO might also need notifying - phone and ask them.