Try and avoid dot and dab. Could the wind post be a T section, top oh the T in the cavity and a 100 web in the blockwork forming a movement joint, then expamet and wet plaster it,

The worry I have about dot and dabbing over a parge in this case (I would never normally even consider it) is the parge cracking but this being unoticeable due to the plasterboard. Is this why you wouldn't do it here Tony or is just a general loathing of dot and dab?!

Good to know I'd have a wind post there then

I had windposts in my end gable but they were in the cavity and wall tied to the masonry block on both skins of the cavity wall. The SE specified them as one length and from the foundations up were ca 7M long. I had a grave concern that as we were building the wind would move the posts on block work that had not already "gone off" . We braced the posts very much like you would a yacht mast and it seemed to work. The brickie had not come across windposts done this way before, his previous experience of them was in storey size lengths and flanged, bolted together as construction progressed to that level, so that the leverage effect of the wind on the length did not affect the courses laid. So maybe something to consider if yours are in one piece and are in an exposed area to do as storey length sections. Being within the cavity cold bridging was an issue but the air barrier was not. I note you have a 200 cavity so it looks like you could put the posts in the cavity within the insulation thereby sacrifice a bit of insulation for ease of forming a robust air tight barrier. Is there a reason why they are in the wall and not in the cavity? As an aside my SE stated had the architect made the opening for the bifolds 4 m instead of 4.6 M then we would not have needed wind posts. So by redesign could you get rid of your posts?

That sounds like thermal bridging to me

From my single windpost experience: architect specified a nice elegant U-shaped ancon one, to sit in the cavity of our 100-200-100 block-cavity-block wall. Builder inquired and found them out of stock, on 6 weeks delivery, so instead he went for a rather long piece of steel box section to sit in the cavity, hugging the inner leaf. Masonry anchors embedded in the inner leaf were bolted or screwed into the box section, and I took to the job of sculpting Celotex to surround the windpost as to limit the thermal bridging. Where the post met the roof, restraint straps were used to tie the glulam ridge beam to the windpost. The post was cut to length so the top layer of roof insulation would go over it.

The advantage of such a wide cavity is that although a beefy 80x80mm (?) box section was used, it left 120mm to be filled with Celotex between the outer leaf and the box section, which, as it is tied to the inner leaf, is part of the warm volume. The U value of 120mm of Celotex is quite close to 200mm of mineral wool, so no big steps in U values either.
Initially I thought the box section was going to be a disaster, In the end I was happy with the solution although a few more details could have been obsessed over from a thermal bridging point of view.