I have just put a twin wall system up with the kinks in it as you describe. The problem comes with the need to get an air tight collar around the system before it leaves the roof. I haven't worked out the solution yet so any ideas would be welcomed.

When I put mine in, I built a sealed box entirely out of fireboard right below the final opening through the rafters. The box enclosed the final angle rafter support. The sides and bottom were closed in, and a circular hole in the underside sealed against the twinwall with fireclay gasket. The TOP of the box is of course open ventilated to the sky because the flashing kit to the tiles is just a sheet of steel with plenty of draughty holes all around it. I figured that any heat in the box can escape easily straight up through all the gaps under the flashing. Lower flue section is properly ventilated through the house with all the part J regs. Exiting it to the exterior with a 50mm gap all round seemed to be just plain daft, and just would produce a secondary chimney!

The part of the flue in an upstairs bedroom has a vented and boxed-in enclosure with large high-level air exits to the room so it gives up it's heat internally. (I put mesh over the vents to prevent kids etc "posting" stuff in!). It's all the heat that bedroom ever needs, it pours out (albeit at at ceiling level).

Inspector seemed OK about it and passed my installation for building regs. It's not ABSOLUTELY within the rules (which really do seem to expect you to throw away perhaps 20% of your heat to the sky).

I added a picture attachment of during installation, let's see if it works

'fraid I couldn't see your pic either....

Justin, thanks for your solution - will read it slowly and come up with a drawing but please try again with the photo.

Edited -bump - worked this time

Thanks very much.
AA