Mr Punter

That is a very well designed and executed car port which looks like it will stand the test of time, as well as the Scottish climate. Nice job.

Screwfix have a Trade pack of Torx screws on offer at 60% discount. 1400 screws for £19.49. https://www.screwfix.com/p/turbo-tx-tx-double-countersunk-multi-purpose-screw-trade-pack-1400-pieces/771vv I think @ToughButterCup is a fan of Torx.

Easifill is a good filler /jointing compound. Sandpaper to finish. Some joint compound goes off very hard and is difficult to sand.

I think the cause was linked to a tree in a neighbours garden. Neighbour not cooperative.

If this is a rental you could just fill the cracks and redecorate. If the roof is letting in water you will need to get it sorted. My mum has a rental in Swindon with subsidence cracks. Five years on and not much progress with the insurers.

You can either rip it out or just leave it. You may be able to leave it for ages without it being an issue. As an interim measure, you could aco round the edges as you don't want any water build up in that corner. You can get cheap stuff at about £12/m and dear stuff at £40/m. Easy to fit.

There are a few on that bottom row that are not sitting nicely. That is quite a kick up.

Is the earth under the joists damp? It needs to be completely dry. Is there any ventilation to outside? Ideally there should be airbricks either side and a crossflow of fresh air. Can you rip out the timber floor, insulation and screed, or is that too disruptive?

Another vote for make your own.

In the past I have put in temporary floor decks with 18mm T&G OSB which was later binned and replaced. This was with treated timber joists. As it was a 3 storey building and a bit fiddly there was no chance of getting up and in the dry quick enough.

I have seen mist systems specified in student hmo kitchens. Expensive but effective if you only need to cover one room. For whole house cover a sprinkler system is far more cost effective.

Brickwork looks the nuts. DPC / gas membrane anal but in a nice way. I bought sand from Cornwall for the colour. £80 a pallet haulage but the alternative was awful.

Cheeky (expletive deleted)ers. DIY job. What ventilation is under the floor?

Very much my taste in landscaping.

Raising the lawn 500mm may cause planning issues.