Mr Punter

Foil backed gives you a vapour barrier and can be useful for ceilings underneath cold or "hybrid" flat roof where condensation may be an issue. Mostly I don't use it. You can paint normal plasterboard with paint that has low vapour permeability.

For the smaller openings a separate lintel for each leaf should work OK. They may need propping during construction. For the big openings you could use a course of Marmox on the inner leaf on top of the steel which will help a bit. @tonyshouse did a wide cavity (400mm) masonry build and there is some info at http://tonyshouse.readinguk.org/mitigation-of-thermal-bridging/#more-247

You can build in blockwork and leave it exposed indefinitely as long as it is properly jointed and there are no gaps in the mortar.

Err no. When they do the air test they mount a bfo fan, normally in the front door opening. It will be good for over 500l/s compared to a bathroom extract at about 20l/s. A big MVHR unit will manage 175l/s but most are less than 100l/s.

I had similar high pressure up to 10 bar and had a prv fitted just after the stopcock indoors.

It sounds like you have excluded ICF. Timber frame can pose some issues with differential movement against a masonry structure. Brick / block may work for you. Can you get a builder to do the whole thing up to weathertight? Might save a load of hassle.

Yes. Either screw in ties or furfix.

What is behind the batten? If you are lapping the boards you will need a much longer fixing. If it is timber behind the batten, go 50mm or so. You may find this quicker with stainless ring nails and maybe a nail gun with a no mar tip.

With some ICF the walls can lean and the blocks can leak during the pour.

Nice. Did you do the whole storey? Any issues?

If all of the wall is on your land they do not have the right to attach anything to it.

I have a similar setup where 2 fans share the same duct. I used Airflow Icon 30 mixed flow fans, which have an iris valve and this prevents cross contamination and means I am only extracting from the room where the fan is running. They have lots of options for humidity, delayed start, motion sensor etc. You may need a larger diameter cutout for the fans. Shop around as there are some deals to be had.

The metered side.

You must not run different services in the same duct. Nor can you run them in soil pipe etc. A total no-no. You are, however, allowed to run services in the same trench.

I have just done some gas pipe MDPE straight in the ground under a ground bearing slab.

I have watched someone screed a very uneven floor. They built up level around the edges and then filled in towards the middle. The result was perfect. I would not dream of doing this myself as it was clearly a very skilled job. I have screeded sand for block paving but doing a whole house is a different prospect.

If you help carry them down and back up you will save any damage and he will be less grumpy.

Some manufacturers have shower valves with reversed hot and cold for just this. I had to use one where our plumbers messed up and there was no access to the pipework.

If you have the space you may be better building a retaining wall a metre away from the structure.

If you are using bulk bags, buy all-in ballast instead of separate stone and sand.

You can easily reach PH standards with beam and block. You will not need any rebar or concrete / blinding / type 1. Given the gradient it may make sense.

I have a groundworker who is obsessively tidy. I pay over the odds but the site looks really tidy every night.

Another vote for beam and block. Try to keep the beam lengths fairly short as it makes the floor feel much more solid. Once the beams are craned out it is a diy job to finish. If you do get a crane for the day you can use it to distribute packs of blocks and bricks around the site.

It could make a useful soakaway if you are not on clay and you want to drain surface water.

Why not just excavate down to solid rock or 500mm, whichever is less?