Redbeard

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?

There is no discussion to be had. Not least because of that 'droop' - never mind what's hidden behind the once-wet plasterboard - they seem to be proposing to hand it back to you looking worse than it did when you bought it. Say no and stand your ground.

....and ended up with 50mm PIR (only, as far as I read) in the walls? Since SAP for new-build is a whole-house affair (with back-stop - worse - U values for each element if I recall correctly) unlike refurb, where it is 'walls must be x; roofs must be y' etc.) I am not saying 50mm PIR would not 'hack it' but it sounds like half or less what I'd have used. What was stated in the archo's dwgs? My U value calcs are only rough ('fag-packet' rather than software) but the lightweight blocks must be a lot better than the standard blocks in my default to get 0.25W/m2K. Was there really less than 50 PIR (say 100 rockwool) spec'd initially?

But I am a pessimist! And someone for whom wonder-glues rarely do what they say on the tin/cartridge/tiny tiny little tube...

My thoughts too, though it depends on how many pieces the 'piece' is in. Many people have a lot of success with epoxy. If you have the piece as a whole piece you could try drilling holes for, say, 3 dowels in the main cill, bed the dowels in epoxy in those holes, and drill corresponding, perhaps slightly oversized, holes in the broken piece, clean and 'butter' the faces with epoxy, then breathe deeply and and press together. If you are lucky the reveal could help wedge it together. Optimist's view: Done! Pessimist's view: (Just refuse to entertain that view!)

While the extension is incomplete it is the best time to chase and resolve the leaks. They will bug the hell out of you in future if you don't. I agree with @Russell griffiths that the membrane on the sloping soffits looks like standard thin poly VCL. There's a small chance it could be something else but that green is suspiciously indicative of 'Bargain Basement VCL'. I'd like Intello too. Please do not give up on leak-finding! I assume the walls are leaky too? But even if they are not, as far as the roof goes you want (a) the right membrane and (b)someone to make sure it is as tight as a tight thing from an air-tightness POV. If the reason you have given up with a/t is the difficulty of finding the holes have you thought of making a basic 'blower door' (old desk fan or car radiator fan in a lump of ply) to tape into an opening? If you de-pressurise the room you're checking then, certainly in this weather, you will feel cold air rushing in. Arm yourself with good-quality a/t tape and you're off. I am sure someone(s?) on here has posted pics of such a home-made fan. Found it: 1st post on p.4 of this: Good luck. Do not give up, and please do not let them begin to plasterboard till you are happy.

Yes and no. The spare tiles were in case you cracked the existing when drilling. The 'platform' could be made of anything. 9 or 10mm ply or cement-board cut slightly undersized so that you can 'point up' with silicone. Equally if the tiles are c 10mm and you like cutting tiles, use tiles, again undersized and pointed. Re the fixing of the cistern, I have a sneaking feeling that one of mine (they are all close-coupled) never got fixed to the wall anyway, with no disastrous effects. I'll go and have a look later!

I can't see infill of 10mm looking awful, so go for that. I guess you may have to plug and re-drill the holes for the cistern, risking tile cracks, but (unless I have missed something) that looks like being your worst potential problem. Do you have a couple of spare tiles?

But it easily could with a heavy, out of balance thud. The proper fixings aren't expensive at all, allow you to tighten it fully and I'd think add some hygiene advantage too. I am wholly in agreement with you. I was not suggesting that screws-only is good practice; simply that it doesn't always go wrong - immediately. The plastic 'hats' (less 'top-hat' than the description I gave) are the right thing.

I have seen 'upside-down top-hat' plastic washers used, but equally I have just had steel screws. Not had a pan break yet...

I may have misunderstood, but if 40mm is limited to 3m, how is your 4.5m run (with a 90 deg bend in) OK? And is the length limitation about potential siphoning, or about the need for a rodding eye? If the latter then I can see how you could 'engineer' a rodding eye at the 90 deg bend so that it truly is 3.1m and 1.4m, but if it's about potential siphoning then you still have at least a theoretical problem...? Can you clarify?

'Weld mesh' to me summons up a picture of 100mm squares. This won't be what you mean, I think. Do you mean 'expanded metal'? If so (based on 'mice men' having told me on the past that mice can squeeze through a hole the diameter of a ball-point pen) that may not keep mice out. I was considering it recently re rats. I definitely do not think an extra layer of PIR will help keep rodents out. If they will eat one layer they will eat 2. I am not 100% certain I have seen mouse damage to PIR (maybe they don't like the foil?...) but maybe I have. I would not risk it with the foil/PIR itself as the only barrier below the floorboards.

Definitely not normal. I have only ever fitted one, and cannot remember how the top trim went on, but it was solid when we'd done it. I hate 'instruction pics' with no words. My brain just does not work that way!