On Thursday, 16 May 2019 18:34:54 UTC+1, Andrew wrote:

Oh dear. I bet if you drilled some holes into the cavities you would
find :-

1. no insulation
2. inch thick expanded polystyrene slabs chucked in loosely
allowing air to circulate on both sides
3. Plasterboard offcuts to avoid paying a landfill disposal
tax.

On the contrary; I've just removed the back wall and was actually quite impressed with the attention to detail: well fitting insulation boards, little in the way of mortar snots; clear cavity down to footings. This was the showhome, or possibly the 'office' next to it, and so was likely one of the first ones built hence the builder's perhaps still had some motivation in them!

Sadly its too late to make a claim from the NHBC, who probably
did the building control. Your local authority BCO would have
spotted this and enforced remedial work.

The thought never crossed my mind. From what I can gather from flicking through the Approved Documents (acknowledging that they don't hold exclusivity over what satisfies the regulations) I can't find anything that mandates even today the need for foil-backed plasterboard per se; more just that consideration must be made for vapour control.

I think the addition of vents into the lower third of the flanks
(there are special vents for retrofitting) and maybe a ridge
vent would help.

I suspect that painting all the upstairs ceilings with oil-based
paint to create a vapour barrier would help, but you would have
to move out while the fumes clear. Don't try this without
doing some research.

I won't be taking any action until I know for certain what, if any, problem I've actually got and what caused it.