Mike

This certainly seems to be key problem - the window / door should ideally be positioned mid-way across the (filled) cavity, or as close to that as can be sensibly be achieved, so that no broken metal touches both the inner and the outer leaf. If that's not the case, I'd seriously think about getting it refitted.

Just taken a look and "Boulder Developments Ltd ... consists of three businesses; SuperFOIL Insulation, Ventilation Megastore and Treefloor" (here). Ventilation Megastore say that they "supply ventilation systems"; no mention of manufacturing. There are many units that are designed to be installed either vertically or horizontally, but of course they need to be properly designed to do that. If you're looking for a replacement, the Passivhaus database lists models independently tested to more accurate standards than those required in the UK and those used by manufacturers: https://database.passivehouse.com/en/components/list/ventilation_small

A block-built house will have many small gaps through the mortar joints and, provided you use traditional wet plaster instead of plasterboard, that should significantly improve - your builder is correct. However it's still necessary to seal all junctions ahead of the plastering, and to attend to the ceiling & service penetrations, but it seems that you have the time to do that. If you've not specified otherwise then that would be justified. However 3ach isn't that special* and there's nothing stopping you paying them or someone else to improve on that now. Or, for the best results, read the many topics on it here and do it yourself. For example think about adding a service void to the ceiling. And get the air-tester back either before, after, or both. After plastering, make sure that nobody cuts holes through anything afterwards, unless you approve where and how. *the best post-completion airtightness result in the UK is currently 0.048 ach (at Larch Corner).

It could be any of those. It could be poor frame design as it's very unlikely that all thermal breaks perform equally well. There are plenty of PassivHaus certified aluminium frames that are suitable for 'cool temperate' climates (which includes the entire UK climate) which, it seems, are tested at -5°C & 50% relative humidity (though I may have that wrong). I'd be surprised if they would permit condensation on the internal surface. But I've never investigated their criteria in that much detail, nor installed them. But if the thermal break is inadequate, then condensation could certainly occur on the inside face. It could also be because the windows is fitted to a reveal that doesn't have an adequate insulated cavity closer between inner & outer skins. We'd need more information about the wall to evaluate that.

Does this mean that Intello type membranes are not worth the cost? Should we be looking at cheaper alternatives instead? The principle is to stop air moving through the structure, limit moisture entering the structure, while making it permeable to moisture vapour to allow trapped moisture to escape. Intello & other 'smart' membranes can play a big role in that (if applied with care and if junctions & penetrations are sealed), but do so by being resistant to moisture movement in the winter (when the risk of condensation in the structure is high) and more open to moisture movement in the summer (allowing moisture to escape). That's generally fine, but if moisture is entering the structure via other means - for example by absorbing external rainfall - then moisture levels can still build up to dangerously high levels. For example, I have a case from experimenting with WUFI (internal wall insulation on a north wall, with MVHR to help control internal humidity) where adding no VCL (but using a parge coat) is fine, but adding a regular moisture-permiable VCL leads to dangerous moisture levels, while adding a smart VCL is even worse. That's not to say that either product is bad, just that they need to be used with care - they aren't a magic wand; follow manufacturer's recommendations and ask them if unsure. But if you were to keep wearing it for the next few hours it should dry - it won't act instantaneously.

Maybe it was originally designed for a warm climate (in China?) & they thought that all they needed to do was add a condensate drain to sell it elsewhere.

The Passivhaus standard was specifically set at a level that means that no conventional space heating system is required. The idea is that the cost saving by not installing space heating more-or-less pays for the cost of MVHR and other enhancements required to reach the standard (at least in Germany, where it originated). In other words, you are overthinking. Just add an electrical heater - or at a blanked off conduit so you could add one later - in each bath / shower room. Or even embed some UFCH pipework for extra reassurance, but there should be no need to actually use it, so leave it unconnected.

Yes, unfortunately that is necessary - though even below the limit your copro might require you to use one as a condition of giving permission.

If access is that limited, are you completely sure that it's good enough for the delivery & erection of a prefabricated timber frame? That's certainly possible - micropiles are popular and an obvious go-to method. Helical steel piles may worth looking at - they're relatively easy to install & minimise the amount of excavation. Or, at the other end of the scale, if you can redistribute soil round the site, or remove it via the access road, it may be possibly to excavate a semi-basement and build off a raft, if the slope is the main reason for choosing piles; it may not be cheaper, but you may at least gain some extra usable space for the cash.

Yes, either could be done in principle, and the vertical part of the 2nd one could either be constructed in masonry or timber. However once the roof gets steep like that you'll need to check out the maxinmum inclination of the pantiles; either slate and zinc could be used instead & would fit with Parisian style. You'd also need to check & enlarge if necessary the capacity of the left hand guttering, as it will be taking much more of the rainfall due to the longer slope.

No glue required - they're just cut a little over length so that they gently wedge themselves between the walls. Short lengths will stay in place until you put the studs in place, or you can slide the batts behind the studs, or a mixture of both - slide just one end of the batts behind a stud, put the batts in place, then the rest of the studs. It depends on the situation. In this photo there wasn't much space to slide them behind the studs so, as there were walls either side to hold the batts well, it was easier to fix the studs afterwards. Plasterboard and gypsum plaster are vapour-permiable, so no problem. Vapour won't move through them over the course of a few hours, but it will over the course of days, which is enough. But but it is necessary to use a vapour-permiable paint over the top - see https://forum.buildhub.org.uk/topic/41776-breathable-paints

One thing that would help is to identify are the location of load bearing walls that can be used to reduce the spans, which will make timbers smaller & cheaper, and to think about how using them as supports them affects your layout possibilities. If you want a clear span then you're looking at steel (thinner) or (better for thermal resistance) glulam beams. Yes, it is. It's too long since I studied the the topic. @Gus Potter may have some recommendations? And having a good summer bypass on the MVHR unit intake air - one that detects when summer external air is cooler and automatically passes all (or most) of the air around the heat exchanger, rather than through it, when the outside air is cooler. Yes, sounds like a good plan, if that fits your room layout

+1 Not in principle.

This part - it's now shallower than before. That depends on their minimum pitch - you need to research that. My guess is that they're already close to the limit.

Your roof is already fairly shallow, so a key question is whether or not the tiles can be laid at an even lower pitch (I'd guess not), or if there's another tile that can. Otherwise you could go to zinc roofing, which can be laid at a very shallow angle & would be in keeping, but which would cost more than tile (and tend to absorb more heat from the summer sun - so add plenty of insulation). I'd guess that it may be possible to reinforce the other half of the roof, rather than replace it, so subject to the zinc cost, overall I'd guess that this may be a cheaper option. Yes, I would expect so.