Mr Punter

A VCL is impermeable to vapour and water and can be fitted to the inside (warm side) or the insulation. This reduces the risk of warm moisture laden air migrating through the fabric of the building and possibly condensing within the structure. A breather membrane is rainproof but allows moisture to escape and is normally located on the outside (cold side).

Wow - looks like a crime scene!

A fly screen mesh is recommended, although if there is sufficient ventilation up through the cladding it does not provide a good environment for bugs and flies.

You should have vertical battens fixed to the timber frame studs, then horizontal battens on top to help create air flow behind the cladding. A is better in your example as you need to make sure any water is directed away from the window head..

If both the FIL and you will end up with houses worth £500K would they consider gifting you their house? They could remain in residence but you could apply for a new mortgage on their house to finance your build. In the event that they live for seven years, no IHT is payable. There is no SDLT either. If they require Local Authority care they won't need to dispose of the house to fund it. It may be a bit controversial, but I think it may be worth looking at.

Not as a structural upper floor, but you can have a screed or use ScreedBoard overlay.

I doubt you will get a decent ring beam without form work. Have a look at

Although you mention some kind of screening, I think the house will suffer badly from overheating, especially in the afternoon. This is best designed out now, as it is very difficult to deal with later.

Brick clad first floor and timber ground floor is unusual and timber is not great near to ground level. Is there a reason for this choice?

Powder coated aluminium? Pre patinated zinc (expensive)?

Afternoon @joth and welcome. I am always interested to read what people are doing with existing buildings, what are the challenges and what solutions are being explored. You may want to look at doing a blog at some stage, as it keeps all the details of your project in one place.

I hate doing this sort of job, so Mrs Punter is assigned to the task, equipped with https://www.screwfix.com/p/zep-commercial-sticky-stuff-remover-1ltr/8648x#_=p I think it may be easier for you to unscrew the plasterboard so you can get to the tape more easily. You could use something like https://www.screwfix.com/p/no-nonsense-window-scraper-3/56292#_=p to help, but careful not to gouge.

I have found limestone is a PITA to keep clean. Have you looked at the porcelain alternatives?

We have lift off hinges that came with the doorsets. https://www.ironmongerydirect.co.uk/product/royde-and-tucker-h101-hi-load-lift-off-hinge-100x88x3mm-right-hand-zinc-plated-pair-990168 I guess the doorset people get a good deal.

As above or you could write a polite letter...