Jenki

That's probably the best thing they did for you, you wouldn't modify a rafter for a vent. The Timber work is poor, GRP messy. not great

You can get single and double Adjustable bends that might help, also you can get tight 90 Deg bends if space becomes tight

Possibly Yorkshire board cladding

I found a "Eco" treatment, that is water based, basically a sachet of crystal substance that you mix with 5 gallons of water. It must chemically react with the Larch, as you can see it change colour when the sun hits it as your treating it. It was a little more Brown than silver initially but 12month in the cladding even on the North is uniform and silver, and does look like its been here years (A look we were after, didn't want the Orange fading to silver in patches look ). I think it only cost £60 for the whole house.

I love the silvered effect of Larch, and you can even buy very expensive treatments to speed up the process. MY tatty old house will never need a coat of anything in my lifetime and no offence taken. Sarking 6" and Wall plate 4" boards in 4.8 m lengths treated are the most cost effective solution I think the 6" boards are £6 a length. I've clad an external shower / WC block and painted with shed paint to good effect.

UFH is different from radiators, it doesn't need to be a fast reactive heat. Well insulated air tight houses don't need rapid heat transfer. The heat from the slab (assuming very good insulation levels underneath) WILL heat the house, if it take 4 hours longer the first time the heating comes on so what? save your money and worry about something else

having a pressure gauge to check is a nice reassurance, I filled mine with air to 1 bar, be prepared for this to rise when the concrete heats the air :). looks like yours is pressurised already? The pipe is tough and as long as you keep excited labourers with sharp shovels away you should be fine. Secure the XPS or it will want to float away. any tape that will stick to the XPS will be ok to seal the pipe. Personally not keen on zip ties to secure the pipe (as these can snap with a bit too much eager poker vibration) especially the small black ties. The two black ducts try to get vertical possibly tie to the manifold board, it would do my head in once finished and the pipe come out of the floor at an angle. Cover the manifold otherwise it will get splashed with concrete from vibration. I used offcuts of insulation, its was easier to get out afterwards. IMHO wouldn't hurt either way.

As you say you can layout very accurately with the 3-4-5 rule and a knowledge or triangles. The problem with tech only approach is when the operator doesn't have the skill to gross check for errors.

For me it would be cost. £30 for the tapes and away you go, my time was free. That said I love technology being used in this way.

With 12 acres to play with this was never an issue for us, but I can guarantee no mater where you stockpile it for "later", it will need moving to do something else ,before you want to use it. 🤪

@nod very impressive 👏. What area or Preston? We lived in Longridge for 18 months before our move North.

Ours just under 90sqm was £11k for reference

D4 PVA will work, but I'd go for 30 min polyurethane. If it foams out of the joint let it cure and trim with a chisel, don't be tempted to smear it when wet.

Is the whole 6m length exposed? With glulam and the visible horizontal joints any decent joiner /competent diyer will sort that no problem even with shorter lengths of timber the joints should become invisible.

Don't let them publish the figures until you are happy with the results. Maybe get a preliminary test done to assist your build..if it hits the required numbers ask them to formalise it.. otherwise do some rework. The worst case for you would be less than 3, ( this should be easy to resolve, by opening a window handle after the tester has checked it's closed😉)