Mr Punter

When the plans are submitted to BC they will do a plan check to let you know if they comply. I think that to comply each room either leads directly to the protected staircase or is on a corridor that is part of a protected escape route.

Wednesday looks hot as hell, so worth avoiding.

Just get them to hover over your tray and crap the contents out. It is a bit messy but it will be fine. Water dries up, concrete sets and can be broken up.

It is self compacting, not self levelling. No need to tamp but you need to place it to the correct level. Do you have a boom type pump lorry?

For concrete I use a sheet of 18mm OSB and some 4 x 2 to form a tray, lined with polythene. Let the concrete go off, then break it up and use it as hardcore.

Yes you will fill the gap between the rough edge and the extender.

Yes, the extenders are the way forward. Shame they were not fitted pre-plaster, but at least it gives something to fill to.

Your biggest challenge will be to get the concrete in the right place, as once it is poured you won't be moving it. If you can get the lorry near all the trenches, fantastic. If not you would do well with a concrete pump - either boom or line type. You will need minimum 3, preferably 4 people, plus the pump guy and concrete truck driver. Work out exactly what you are doing before anything turns up. How will the concrete lorries get in and out, traffic management etc. 6m is very wide to screed to. Can you get some pegs in the ground as levels? If there is a screed layer on top, the accuracy of the slab doesn't matter too much.

I have only done these in flats.

Not me. I would spec it on external render though.

As suggested by @ADLIan and @IanR, it is probably the thermal bridge calculations that are the issue. You are far better off using existing calcs than having custom ones. The default psi values are unbelievably negative and these are probably the ones that were used.

Bed joint reinforcement (the train track stuff) above and below the openings may have helped. I have only specified it on rendered walls or brickwork feature panels like stack bond. Mesh in the render also helps. You best option now is filler.

I saw these people the other day: https://anthonygold.co.uk/service/leaseholders-and-freeholders/conversion-to-freehold-enfranchisement/ May be worth another quote. Are the other leaseholders on board?

It looks nice and tidy. Did you concrete over a pipe there?

I know someone who used expanded clay fill, then concrete slab with UFH. It was a considerable depth. You will need a SE input obvs.