Mr Punter

Yes use treated timber. You will be able to sit the window on it. You can fix to it as well if it is OK to screw through the frames.

I think you will need to cut the plaster coving square and scribe an infill to the ceiling. They will need to do the ends anyway so hopefully will be doing a decent job. If there is a cornice on the wardrobes (I can't really see from the photo) it may need to be removed.

No, you can use timber. You may need 38 x 63 cls to go round the opening between the blockwork and the SIPs. Fit them before the blockwork goes in.

Yes, get a proper SE to inspect and propose any remedial work. BTW intumescent paint goes really manky in damp conditions.

I think @Spinny and @ProDave have got this. Dry trap / stuck dirgo / open end on pipe. Check anything connected to drains (dishwasher, boiler condense, shower, bath, washing machine, MVHR etc.).

I often see architects show stairs that do not work as a standard timber staircase. Your architect drawing did not show any newels or handrails.

Wet mop and sweaty feet do not bring to mind ideal conditions. I think you may need to take up a section of flooring or make a hole somewhere inconspicuous to see what is going on.

Are you fitting this yourself? You may need to have some components supplied loose. If the wall is already there you will need a fair amount of wood butchery. The winders look odd. I think there should be minimum 50mm going on the short ends, so #6 would not work.

It would give a much better fixing if you could fix the counter battens through to the purlins. Unless you go through the t & g boards I don't think it will be strong enough.

The £1.80 per hour was for a 7kW charger, based on standard tariff electric at 25p per kWh.

With a home charger it will cost less than £1.80 per hour while it is plugged in, so £30 would be difficult to reach unless they turned up in a car with a 120kWh flat battery and hung around for 24 hours.

Blimey. Do you charge if they have a drink or want to use your loo?

The advice from @Russell griffiths was good. A labourer would be useful, but not if they don't know what they are doing. You may as well get them to mix (with a mixer) rather than have it delivered. Make sure is it just moist enough that it holds into a ball if you squeeze it, but not wet. You are best with a polythene layer on top of the Celotex. A laser level may be helpful. A long aluminium straight edge to screed with. Use kneepads. I wouldn't fancy doing this myself as I would be too knackered after the 2nd hour.

If you want decent insulation the Dritherm 32 are the ones to use. In the great scheme of things will not make much difference. Have you priced the actual difference in £? 150 cavity is simple as it works with digger bucket for foundations, standard wide lintels and standard long wall ties and standard cavity closers. Quite why you would build in brick on the inner leaf I have no idea unless you want an exposed brick inside, which I would not recommend as it will cause big problems with services and airtightness.

It used to be possible to use pir in EWI. Post Grenfell I imagine most of the certifications have been withdrawn. Get in touch with a render firm (weber or whoever) to see what they suggest. Could be tricky.