Mr Punter

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.

If the cavity does not need ventilation you could fit a DPC cavity tray around the base of the plinth bricks linked to the inner leaf and fit the mini weep vents at 900mm centres.

Well the good news is they haven't tried to cover it up. Get a full survey and report. Make sure it includes roof condition, timber condition, damp, dry rot and condition of brickwork.

So is this an insulated masonry cavity wall?

You need to confirm with the contractor that they will immediately spray the curing compound on the finished slab. If the water evaporates from the slab too quickly the concrete will not properly cure. That said, the house will not fall down.

If there is only 1 layer you will be fine with the overlaps. If you can get someone to help placement it would help a lot as the sheets are heavy and awkward. You may want some tie wire where the sheets join and allow for a 300-400mm overlap (40x wire size).

On a gable with a raking roof abutment and windows, it is hardly glory work!

Stapling the tape on may help straighten it up a bit. Get it clad ASAP.

Looks fine. I would normally expect to see vertical stapled on tape to identify where the studs are.

Keep the higher bits as close to the surface as possible, especially of it is path only. 300mm min cover is fine and you could do less at a pinch. Use the mini shallow ICs at the sides and back and for corners where you will add a 45 either end. Avoid clashes with the surface water system. Make sure the foundation is low enough where the pipes cross at the front. This can just be short sections.