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I'd also be reluctant to do anything drastic and expensive, hopefully the whole floor will float but stay flat and you'll get used to the creaks. One possible explanation might be a lack of expansion gaps around the edges of the room, so if you do ever need to investigate further I'd start by taking all the skirting off before doing anything to the floor itself

Ours needed to be stuck down with Rapid Bond, a polystyrene safe spray. I'd be very surprised if any polystyrene boards could be dry laid, they're really weak, just a few mm, where they're routed out. Can you ask the board manufacturer? I'm guessing you only have a few mm of leveling compound over?

Easy to find 930W for £499 on eBay. No link as I've no idea if they're UK approved. I'm sure there'll be plenty of questionable offerings at low prices soon enough

I could've been clearer, I was actually thinking in the context of @Gus Potter's suggestion... If one end is fixed to the exisitng wall and the other end is built around a post slotted between the two steels, you'll avoid any wobble. I don't think a solid baluster wall will look bad at all by the way!

What's the material infilling between the two steels? Could this be removed so you could slot/wedge timber between to effectively add a newel post?

Yeah ours came welded from subframes, but Eurocell can also supply slot in corners if you make them up yourself, shown here https://www.eurocell.co.uk/library/download/link/id/23506/

Having owned property of similar age and condition I don't see anything that would greatly concern me. The house probably has minimal foundations, brick corbel plus maybe 100mm concrete if you're lucky, but has lasted over a century including a mix of exceptionally dry summers and very wet winters with what look like minor cracks. I think it's worth getting an SE to look at your specific concerns (the left E pic most notably) but I'd be careful not to get pulled into a lot of expensive remedial work for limited benefit by the first less than scrupulous builder you speak to.

Yeah we had to submit a NMA for this, no problem getting it approved but an annoying hoop to jump through plus the cost of application

Ours were EPS filled. Worth mentioning you can buy all the bits from Eurocell and make them up to size yourself, which may save a bit although I don't think they were too badly priced ready made.

We used these for 'normal' sized openings (say up to 1500x1500) and they worked really well. Ours were from subframes, made from (cavalok big blok I think) 150mm closers. They were very substantial and came braced at top and corners to stay square, worked well as formers for brick and block. They have slide in ties for brickwork so feel very built in. Totally different from the floppy bits of plastic and polystyrene of a basic closer.

I'm a fan of Hep2O but that advice is direct from the manufacturer, so probably best heeded, Presumably doesn't like to be hot/expanded for too long

(You could probably do the same from a single piece of lead, working it around the outside edges if you're careful)

I'd think in terms of building the world's smallest flat roof and I'd build it on the ground then drop it into place, bonded to the bricks with a decent adhesive. Get a piece of OSB that will overhang 50-75 on three sides, put something like your arris rail around the underside edges to create a drip, then dress the whole thing with your roofing membrane, making sure to leave an extra strip that you can lap up the wall once it's in place, normally you'd have a cover flashing over but your main roof membrane may over lap to do that job

Did they also deburr it for you?

I'm cautious/sceptical because the site doesn't provide any ownership or business details, and so wouldn't want to give my credit card or personal details or suggest anyone else does. You still haven't said how you found psiclops, considering it's pretty invisible on Google for example, and from the level of detail and content in your posts, one might infer you may have some connection with the site.