Redbeard

Agree wholeheartedly. Always a very huge way outside my price range. Ultimately had my V twin the across the frame, and no chain! Please forgive the M/C-related hijack!

In my limited experience It was b----y hard to do an 'invisible mend' on patches like that. I hope your patches don't 'show'. My experience was doing it myself. If someone else is doing it for you then they have to try to get the 'invisible mend', and arguably re-do it if they cannot. Again, hopefully the pictures are not 'telling the story', and it's not what I think. EDIT: @Conor must have posted his while I was 'composing'. He got an 'invisible mend' - things can obviously be more positive than I suggested. Good! And good luck.

Hard to tell from a photo, but based on what I seem to see yes, I think I'd be a bit concerned. The 'bald patches' look like... well, bald patches, where the grit has not spread evenly. But maybe I am seeing what's not actually there... Are the 'bald' patches smooth?

Silly me! Posted asking where the Duke was, without looking at the 'thumbnail' pic! Too little of it to guess which model, for me.

Rather than metal straps we use to use (for a range of timber buildings) 50 x 50 sticks fixed to the face of the bottom beam and purlins and then screwed sideways into the rafters. Don't have a pic to hand.

No, but have you tried Allan Brothers? Never used their alu clad (which are imported) but a client has used several 'batches' of their UK-made timber windows and was v satisfied.

Good call, but the use of squirty foam at all will be governed by the answer(s) to the ventilation question posed previously. Squirty foam should only be used in well-ventilated conditions which, in my experience, often do not exist in sub-floor voids. Also having to use the gun at an angle increases the risk of sputtering and 'bounce-back'. ('Sputtering' when gun delivers liquid propellant instead of foam because the canister is not vertical, and 'bounce-back' when that liquid his a joist and splashed back, under your safety glasses (which you wear because goggles steam up) and you go to the local eye casualty. Don't ask me how I know...

... or is it just the bottom flange of a 'box and tray'-type lintel?

@Iceverge, I definitely agree with you in principle, but the part of the world you are in (and the size of the job) can have a big bearing on whether you'll get a cellulose blower out of bed... I've known colleagues struggle for large extensions, let alone one floor. I don't know if it still is, but Wales used to be a good place to be for cellulose. Is Ireland similar? I don't know where @Rocket Ronlives, but it could be pivotal w.r.t cellulose. I've talked about blocking 'unintentional ventilation', and you @Iceverge, have suggested cellulose and membranes, but I don't think any of us yet has (in this thread) explicitly stated the need for excellent cross-ventilation below the insulated 'sandwich'. Yes, getting rid of warm moist air from the house entering the void is good, but if *any* vapour-laden air gets into the void, which is now colder because of the insulation, there's a potential chance of condensation occurring on the bottom of the membrane. Good, unimpeded cross-ventilation (straight lines and diagonals) is a life-saver for suspended floors. Vents in one elevation only mean you have 'ins', but no 'outs', and the air 'short-circuits'. Edit: Come to think about it @Iceverge's idea of adding to the bottom of the joists could be done in home-made OSB/plastic insulation/OSB 'fillets' to cloak the thermal bridges of the joists. For example, perhaps instead of 50 x 50 PSE, 2 x lengths of 12mm x 50mm wide separated by 25mm PIR or EPS/XPS and glued up with waterproof glue. (I used EVA nearly 40 years ago and the 2 elements are still stuck together!)

Heat is lost by conduction (in this case through the floorboards, which you aim to limit by insulating - though note there will still be heat lost via the joists) and convection - air-borne heat loss. Yes, you want the floor structure air-tight, but water-vapour permeable. Water-vapour-permeability is what energy geeks call 'breathability'. You need to sort both sets of losses.

If the house is Edwardian there is a fair likelihood that the skirting boards were hollow-fixed first, then the plaster carried down to the skirting. If this is the case one of the few ways you'd get the floor airtight at the perimeters while leaving the skirtings on would be to pump air-tight foam up behind the boards from the bottom. This has the disadvantage that you are very unlikely to be able to use a foam gun safely (i.e. with the can upside down). In my relatively limited experience if you attach plastic pipe to the nozzle you do not get enough pressure for the foam to 'stick'. The alternative is to find something with the consistency of 'sticky chewing-gum' and ram it up the back of the skirting from below. (You can see why foam was my first thought!). @Russell griffiths's suggestion to take off the skirting gives you a chance to get really good perimeter air-tightness.

Talking of reading the photo, is that a notched 'plated' purlin bearing on that rafter/Truss? Or is it some sort of decorative 'trunking'?

I should probably advise a condensation risk analysis for IWI on top of CWI. WUFI is good but Glaser is at least indicative. My reason for insulated stud (in my mind a glued laminate of, say, 12mm OSB, xmm plastic insulation, 12mm OSB) is to reduce thermal bridging. Metal stud leaves a bridge all the way through the sandwich, but is a narrower highly-conductive thing against a fatter less-conductive thing. So the end result may not be so different. I don't know!

I think , off hand, you'd want insulated stud, Intello, secondary (say 25 x 50) stud, plasterboard, but have a look at Pro Clima's site. Ecological Building Systems (UK importers of Pro Clima) may advise too. Note I was writing that when @Oz07's post came in. I like Intello as it acts as a VCL but, if any moisture has got where it shouldn't, can become slightly vapour-open to let it out.

Depends what you are putting it on top of. If it's gypsum painted in vinyl silk then your hopes of breathability are arguably less well-founded than with a lime plaster with breathable paint (opinions vary!!). I would make insulated studs and I'd use Intello membrane as VCL.