Mr Punter

I have used plastic lumber for below ground. Over twice the price of timber and may not have good fire rating for cladding but it does not rot.

Yes, 50mm concrete screws are your friends here. For a 7.5mm screw drill a 7mm hole and clear it out a bit so the screw goes in tight with an impact driver but does not lock up before it is home.

One nail per strap looks a bit stingy. Often see screws and plugs or masonry screws. No issue with zipping off the surplus strap with a grinder and thin blade.

OK in theory but this never ever happens on site, which is why the "plasterboard tents" are notoriously freezing.

In reality this is more likely to lose heat v solid plaster, as it can allow cold outside air to circulate behind the plasterboard. This can be very significant and difficult to resolve. Another failing of SAP.

Unless this is a strict planning condition you may find render is more acceptable to future purchasers and lenders. Probably cheaper and quicker as well.

I don't think you have misread. It looks like a thin coat render onto mesh on carrier board, so probably 6mm thick. God knows what has gone on and what to do. I would go back to the architect and ask for their suggestions.

On the drawing the flashing is not long enough to cover the top row of tiles. Having a roofed canopy addition on a timber frame with a rendered finish is always going to be very tricky to sequence and detail. It looks like the architect put this in the Too Difficult pile and left it for "others" to mess up.

You could use 82mm soil pipe to gain an extra 28mm. Plenty of fittings available. https://plasticsuppliesdirect.co.uk/soil-vent-systems/82mm-push-fit-soil-vent-pipe.html?p=2

You cannot join a WC in a Y. You will need ICs. These are best outside the footprint. Why the soil pipe in bedroom 03? As above, you need to work out your levels and take it from there. Design the runs as shallow as possible with a fall of 1:50.

I like medium density aggregate blocks, like Hemelite. They are a bit lighter than dense concrete and they cut more cleanly with the bolster. Easy to plaster and render and can be used underground. They won't help with u values though.

Yes, you are both right. They are so similar to Velfac, plus made in Denmark, I assumed they were the same group.

Yes. I have used their inward and outward windows and sliding doors. I have not seen / used Pure View. The 2 firms are part of Dovista group who also do Velfac and Velux.

I can't see a pumping station costing as little as £1k supply and fit. Good luck when it fails (which it will). New connection or RC slab over pipe.

Option 1 may need a DPM. You normally put a thin sand layer over hardcore to cover the sharp bits before you lay the DPM. Is foam glass expensive?