Redbeard

How thin is thin? My 'go-to overlay' used to be 25mm PIR and T&G OSB floating (not fixed) on top, but I always used 18mm. 9mm might just be OK (I don't know - I have never tried, and of course you don't get the T&G) but I think 6mm over PIR might feel a bit like walking on a blancmange.

Exactly. One more trade to find and wrangle (and worry about if their credentials are not tip-top). Find a good slate roofer and stick to slate. It'll look lovely! (But please don't quote me on that if it doesn't!! 😉). Much more chance of finding a local slater whose work you can look at than a local standing-seam contractor ready to do a small one-off, I would guess.

Judging (only) by a few posts on here re difficulties with quality on standing seam roofs I wonder if you might have things against you - not least a roofer wanting to 'get out of bed' for 20m2, when there are full houses to be done. Or I may be too pessimistic - 20m2 may be just enough to fill that small gap left after the last job...

That sounds OK, then. How about all the other 'potentially weak' areas (junctions, intersections and so on)? If you've been taping and sealing with a vengeance then you may be OK. I am not sure what you mean by: I think of dry-lining as a whole-wall treatment. Can you elaborate? You say the cavity wall is rendered. What is the internal wall 'treatment' - hard plaster, or dot-and-dab plasterboard? If the latter, is it 'true dot-and-dab', or full perimeter beads and cross-hatchings?

If that means you can feel 'something of a draught' (at atmospheric pressure) from the sockets I think I'd be a bit worried about other areas. Who has it been built by? Yourself with a concentration on air-tightness, or a general builder with no specific air-tightness 'leanings'?

I realise you will probably be there for life, but just in case you ever need to sell, might I suggest that you try to 'buy' that 50mm? A client of mine did exactly that, though they at least knew who owned the land. In the case of a 'shared road' it may not be so straightforward to find the 'owner' to buy from.

I was typing while @JohnMo was posting. His points are very valid. Why rip out if you cannot afford to re-do? If the potential cost is completely unaffordable why not ask for a staged plan?

Welcome. What's the orientation? Is it conducive to solar? Do you have plans for PV? I assume the house is of cavity construction. is the cavity insulated? If money is tight I am not sure how keen you'll be to completely remove the bays and 'plant' new bays on top of EWI. EWI also needs care to avoid the risk of thermal by-pass via the cavities. If you were to go for internal insulation (IWI) you'd be limited as to what U value you could achieve, due to the risk of interstitial condensation. Detailing can also be tricky depending on the internal layout. On the other hand, assuming you do not own the land to the right, I'm not sure how practical EWI would be, either. Ideally you need to find a way to use one or the other, or a mixture of both. If perchance, there are 'viable' (50mm+ and 'clean') cavities and they are uninsulated, start with that, with EPS beads (not mineral wool, in my view).

The same could apply with a double-skin timber frame with much more choice of insulant and a more 'permanent' 'feel' (both of those 2 words pretty vague in meaning in this context, but clear to my old brain).

why ply? Why not plasterboard? Some of my sloping soffits have thin wood-fibre (40-50 year-old), plastered over, and almost certainly as an underdrawing to knackered lath and plaster (This year's job to remove and insulate with 260mm wood-fibre and lime plaster over (yes, had the structural calcs done!). The plaster on the aforementioned thin WF causes it to sag, so it looks cr*p.

What you describe is a hybrid Warm roof - some on top of the rafters and some between. In some circumstances there is a risk of the dew-point occurring at the interface. A condensation risk analysis (CRA) can guide you, but a colleague had a good rule of thumb: If you have two-thirds of the R value above the rafters and one-third below, the interface is unlikely to get cold enough to reach the dew-point. If I remember correctly the lambda of cellulose is (or was when I used it) 0.036W/mK, and PIR is 0.022W/mK. So for 125 cellulose it's 3.47m2K/W (actually less, as that does not take into account the 'intrusion' of the rafters) and for 100 PIR it's 4.54m2K/W, so not too far off but probably not two-thirds/one-third. A CRA (particularly if done in the WUFI software) will be more accurate. Edit: A thought: If noise is an issue why not consider rigid wood-fibre on top? 140-180kg/m3 as against about 32kg/m3 (IIRC) for PIR)?

I would not advise skimming over paper, however well-bonded it appears to be. Borrow a steamer and steam it off. 24hrs seems a v short time to leave a deeper base-coat before skimming. I'd just say leave it longer. If you can get hold of a small amount of lime putty (which lasts forever when kept under water) you can just mix it as required with plastering sand. There are specialist lime etc. suppliers not so far from Oxford.

Do you really need access? What is there to access? Wiring? Plumbing? Ductwork? I am guessing none of the above, so that leaves 'the void'. Project yourself forwards 10, 20 or 30 years and think if the necessity will ever crop up. If you think it won't, don't have a hatch. Worst case scenario is that years hence you have to cut the plasterboard and get deluged with Warmcel. Not the worst deluge in the world.

Congratulations - a big worry off your plate. Perhaps more importantly, is the house (and are the bills) comfortable?

Re the sloping roof, unless they guessed it wrong 1st time (design U value), all other things being equal (which perhaps they are not*)I cannot fathom how they get a worse U value with 150 (as-built - apparently 0.2W/m2K))than with 125mm (stated as 0.18). *Much closer rafter centres and fatter rafters (therefore larger timber fraction in the calc) in as-built than design? So this is to the pre-2022 Regs, then?