Classification help.

[zeebeedee]

Pre cast flue, onto twin wall in the loft to a ridge terminal. Not spilling but noticed when I was redoing the cement seal where the twin wall connects onto the pre cast that there was no connector piece, just precast adapter ie rectangle to circular then twin wall cemented in.

 

[GrahamM]

Its one of those that comes down to your engineering judgment zeeb.
Would need to see it to give mine, but if been sound with no issues for some years and you do not foresee any issues I would be inclined to NCS if all checks ok.
If not 100% then AR to cover yourself.