Mr Punter

Bear in mind that in another 7 years the warranty will be moot.

Well that will save you having to measure! It is now a case of identifying where it it leaking and I can see you are doing some decent investigation. If you can Identify and seal the leaks with, say mineral wool, you will not need to add more insulated plasterboard. Maybe a 100mm hole saw to remove some plasterboard around the perimeter will give you some clues.

Get in touch with the managing agent and kick up a fuss. Get chatgpt to come up with some wording and a plan.

Are you using a heat pump?

I don't think this is down to the door installer. Perhaps look at removing the top course of bricks and replacing with high compressive strength EPS. What is the floor finish?

If I were you I would get a tape measure and some paper and measure in the downlight hole from the OSB to the bottom of the plasterboard. Then measure from the plasterboard to your finished floor and add the measurements. Then go outside and measure from the top of the flat roof to the internal finished floor. If you subtract the external measurement from the internal measurement you should have a rough measurement of the insulation thickness. This will probably be between 100mm and 200mm. If it is less than this the roof may not have been insulated correctly. If there is a parapet around the roof you will need to take account for this in your measurements.

With the polystyrene beads they get mixed with a small amount of adhesive as they are installed, so if you later make a hole in the wall they don't just continue to escape. You would not be able to do this easily.

I think you should shop around for more quotes or just sell the land on. Surely you can buy a top end finished project for less than this?

I would get an online estimation firm to do the take off and quants.

I have had floors sanded and finished in the past. They spend a while removing or knocking in any protruding nails. Used a BFO drum sanded for the main areas and a smaller sander for the edges. The gaps need a flexible filler, not PVA and sawdust. I would get a pro in to do this on a price.

Carefully note the advice above from @ADLIan. If the outside skin is brickwork, a coat of Stormdry may help mitigate. If it is rendered, you must carefully check for cracks and imperfections.

What is the problem? Are they letting in air / water?

You did well to get everything properly masked as that Diamond paint is very difficult to remove when cured. Also rollering a final coat for the walls is good. Tiling looks spot on.

https://www.squaredealupvcshop.co.uk/ do the Hardie cement board stuff.

In reality they won't happen as not practical / cost effective.