Mr Punter

I have fitted a Vent Axia Sentinel Kinetic plus in a house. Simple enough to do but make sure you have decent access for servicing / filter cleaning. It was designed in the loft space but the house is 1 1/2 stories so it is stupidly tight. Doubt the new owners have ever cleaned the filters.

You could use 10mm thick flat bar for the vertical parts. You may even have enough room for stiffening where it joins the box section. You could have galv powder coated steel if the stainless was expensive to fabricate.

We did similar on a place with bris soleil with slate cladding on timber frame. The brackets have welded flat vertical sections either end screwed into the timber frame.

Can you not take off some cladding and screw onto the timber frame, then just cut a neat hold in each bracket and slot the cladding back?

Floor insulation is simple and cheap to install so you may want to go a bit thicker. The screed does not need to finish level with the bricks. You will normally have a 38mm thick sole plate plus the bottom rail of the timber frame so you could go 120mm insulation, 60mm screed and there will still be 46mm of timber at the bottom to fix your plasterboard. The timber frame firm should design the holding down detail. It could be metal straps fixed to the floor or screws and plugs. Ask them.

I understood that the condensate would form as soon as the exhaust air was passed through the heat exchanger and it would drip into the the condensate drain and be directed out. There should not be enough moisture in the now cooler air for condensation to form on the remaining length of duct.

If the neighbouring houses are connected to the same drain and they have vent pipes you could just have an AAV. If not, you will need to vent to atmosphere. If you have more than 1 soil pipe you will need either an AAV or vent to atmosphere at the top of each.

Not with any detail. Just showed a twin wall metal flue I think. This was 10 years ago.

If it is loose it is OK on a nice concrete base but a pain if it is not on a clean surface. Bricklayer will not thank you for mud and stones in the mortar. Even worse if it is used for render. I do find it much easier to shovel from a concrete slab than digging from a bag though.

I used this stuff https://www.stovesonline.co.uk/wood_burning_stoves/Selkirk-flue.html It is twin wall and I used the 150mm which has an external diameter of 200mm.

I don't think that HETAS registration is mandatory, just a way of showing compliance. I have done a metal flue in the past as part of a new build house, all to spec and Building Inspector was fine with it.

As I have previously posted, I think Fermacell is a right bugger to cut and fix. Very clever marketing.

Perhaps you could try the GGF as they seem well regarded in the industry: https://www.ggf.org.uk/technical-information/tech-consultancy/expert-witness/

Have you established if the leak is around the perimeter seal or is it in the window system itself - such as around the glazing gaskets etc? The former suggest prep, flashings, mastic / compriband / installation issues and the latter suggests manufacturer or damage during installation. Can you get in a reputable window firm who deal with several manufacturers and ask their opinion, even if you have to pay for their time?

Can you get rid of the branch on top of the elbow, just have the elbow and lift the horizontal pipe so it all has a really nice fall to the outside? You would probably gain 200mm in fall and therefore no issues.