Redbeard

A whole bucketful of them, I suspect.

Yeah, this is head-scrambling! So the original plan was to take off the roof-covering, insulate with PIR leaving the T&G below the rafters and install further PIR below, yes? Insulation-to-insulation contact is, in my view, always best, but yes, if it was only the T&G there and you could get both layers 100% tight to the T&G there should be no risk of thermal by-pass (cold air in the 'layers of the sandwich'). However with the L&P there you are really going to struggle because of the 'snots'. HOWEVER... Since the L&P is 'under-boarded' and no longer needs the 'snots' to hold plaster to lath you could rake off and hoover out the snots and insulate tight to the lath. If it were me in these (less-than-ideal) circumstances I would primes the sides of the rafters and, after foam sealing any gaps, air-tightness-tape the PIR to the rafter edges. It also took me a while to 'orientate' myself re the 25 x 38 battens but I now see they form the 50mm vent path as per BEIS (now DESNZ) best practice guidance. As @ProDave says, this is not a Warm Roof. You will have a Warm Room below it but the roof structure is a(n insulated) Cold roof.

Please be reassured; 'freehand' in my previous post refers to the guiding of the plane(r). The architrave in question was clamped to the bench! There was simply no width-guide on the planer, but a line I worked to! All fingers intact!

Relative piece of cake. Yes, what you suggest is what I was suggesting. If you have a guide on your planer it helps, but I have done it freehand before.

If it is not too thin at that point how about running a planer over part of the width? It has worked for me in the past.

What does the builder's 'padstone' consist of? Assuming that your Building Regs application is Full Plans they could reasonably refuse to approve any lesser alternative, unless the builder has proof that his chosen solution is equally satisfactory. Better to ask for it to be changed now than when there's a whole wall built off the beam.

@TerryELink does not work for me...

I imagine @Gus Potter or others may be along soon but a first stab from someone who definitely isn't a SE. I take your point about spread and the 'job of the truss'. I wonder if something can be done with steel (or, perhaps better, timber with 'properly engineered' connections in terms of trimming for the opening but still keeping the strength of the truss in terms of its role in preventing spread. My initial thought was a specially-fabricated 'strap' from the 2 neighbouring trusses to 'give back' the strength to that cut one. On the other hand the steel probably gives you a complicated insulation detail at what is generally a weak point (the hatch and opening) anyway. Timber - properly 'engineered' - seems the best bet. Insulation at that point will still have to be thought out carefully.

Good? Well, it depends on your view. Like a garage door? Yes, we can do that. So as not to lose too much space the logical idea is just to leave the up-and-over (I assume) door 'for show'. If it is to open you will need a store-room equal in depth to the part of the door which projects in to the garage. I would build a timber stud wall which is finished externally as if it were 'proper external', including excellent air-tightness - and on the internal side well insulated (with a VCL too) of course. Where will your extractor fan to the front go? Might I suggest a trickle vent above the garage door (and a corresponding vent in the stud wall and have the fan (which does not look particularly pretty) at the rear?

It will be better than nothing at all, but compared to a 15-20mm slot all along the eaves on both sides it will be less than exciting. Also, venting from the eaves gives better 'purging' of warm moist areas. Tile vents part-way up leave areas of 'dead' air.

It looks 'right' to me, not wrong - but what goes on at the LHS? 'Residual flashband' or just some more patinated lead?

OK, fair point. I should have said 'my personal preference is for AT tape' and then backed it up with anecdotal 'evidence' (over quite a lot of years) of some quite good, quite strong and very sticky foil tape and some very weak, not-very-sticky, frankly very awful foil tape which was particularly weak in tension and would split when returned round 90 degree returns. A few bad experiences led to a 'once bitten, twice shy' attitude which I retain for my own work. I'm just off to comb my torso now...

I think the IC analysis 'thought it was OK' as it ignored the 'interstitial plasterboard' and assumed (as all IC analyses do AFAIK) that the VCL in the insulated plasterboard was perfect and taped at all joints and perimeters. The nature of insulated plasterboard makes this impossible. I have a strong suspicion that the musty smell is the paper on the 'trapped' plasterboard 'festering'. There will almost certainly be other issues to sort too, but I think you need to remove the insulated plasterboard, remove the original plasterboard and VCL, apply PIR (if that's 'to your taste') 'in the raw' (not as a plasterboard laminate), tape all joints and perimeters with air-tightness tape (not cheap fragile foil tape) and then batten, fixing through the battens to the TF wall to hold in the PIR. If you are a pessimist (which with water vapour I think you should be) pre-fill the holes, gun in silicone, and wind the screw through a 'silicone grommet'. Nothing is perfect but that is going to be the best chance you have of a somewhere-near-perfect-ish VCL. I am sorry this will not be what you wanted to hear.

Although some have done it I am not happy with the idea of trapping gypsum plasterboard in a sandwich between insulation and insulation. So to clarify, the timber wall is the original, yes? If so I think you should have taken the plasterboard off before adding the IWI board, and only have done that if an IC analysis said it was OK (although I think you did do that IC analysis and it suggested you'd be OK if I have read that right. Obviously the IC analysis software and I don't agree re a plasterboard sandwich!). @Nickfromwales has posted while I have been typing, so off to see what he advises.

PIR is a mixed blessing as many will tell you. V hard to get a perfect fit and as humidity changes a 'perfect fit' may not be one any more. I like flexible wood-fibre, but it is not cheap and will need just short of twice as much as PIR for the same U value. The main thing with UFI is that as you warm up the room you cool down the floor structure, increasing the risk of condensation. Excellent cross-ventilation (no 'blind' areas) can help a lot. Beware that UFI does not always appear to obey the laws of Building Physics!

I have no idea about the cost, I am afraid - it depends on your local 'market' - but I would engage a structural engineer first to prepare a specification. I cannot see clearly what goes on under the stairs. Is that a door or pair of doors to an 'undercroft'? I assume that 'rusted beam' is over a door or doors. Is the space inside the doors a WC, perhaps, or just a store? Is it necessary? Could the structural support be made simpler by infilling the 'void'/store? But then what about the risk of transferring damp to the ground floor room adjacent?

Yes, it will be slow, but it will begin as soon as you have moved the 'pre-conditions' as described above. I'd leave it to do its thing and use the de-humidifier you referred to.

Ah! Thank you. I could not see its base-plate.

If the world is really quiet I can hear my 3 CV2s. If it isn't (and it usually isn't) I can't hear them. Even when I can it is not bothersome.

What is the pipe? How will it be protected and how will it interfere (if at all) with the integrity of your foundation?

No, don't even consider DPC injection!!! Everything worked well (as far as you have said) before the builder put a hard surface above your DPC. All that needs to be done is to remedy that locally. As @ProDave said, probably take another half-block away so that you have a 'trench' of 200-225mm wide. *However, what is retaining the edge of the driveway? Assuming it is a standard block paving job and bedded in sand then probably, 'as-built', the house wall will have been the 'retainer' which stops the sand running out from under the blocks, and the blocks 'drooping'. If this was the case, then removing half a block as the builder has done, and another half as we have suggested, could (if no other method has been provided) lose you your 'edge restraint' and gradually the sand could wash away and your blocks drop 'into the ditch'.* The Building Regs require a minimum 150mm (6 inches) between external ground level and the DPC. That is based on an assumption that rain will 'bounce' 150mm max. No-one has told the rain where I live that it is only allowed to 'bounce' 150mm, so it goes for 300 instead! At very least you should revert to the former (we hope) position, of a minimum 150mm down from the DPC to any surface. Ideally you need a soft surface (the gravel will do), and it needs to be far enough away from the wall that rain won't bounce onto the wall above the DPC. In certain wind conditions, your 4 inch (100mm) gap won't be enough to stop rain hitting a block and yet still being able to wet the wall.

Just for clarification, 'my' parge coats are approx 6mm trowelled lime plaster. I know others here use a brushed-on weak(?) sand/cement mix, but mine is not that. Where old lime plaster in good condition with good adhesion still exists, and there is no impermeable coating, I leave that (probably c 18mm) and simply augment where it never was.

I've done lime parge coat on GF and FF walls and in the ceiling/floor space, tight around the joist ends. Leave plaster to dry thoroughly, prime plaster and joist with air-tightness tape primer, then tape. Usual tricky spots are at joists parallel with flank or external walls, where you cannot get in to plaster, prime and tape. Probably one of the 'gunges' or FM330 in those spots.

Yep, looks pretty straightforward. Good luck!

Agree with @Conor's concerns. If I understand that 'spaghetti' right then the 'resting' water level in the trap will be above at least the smaller of the flexi-wastes, if not both. No wonder the dishwasher isn't draining properly (again, if my 'reading' is correct).