Mr Punter

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 Post Grenfell I imagine most of the certifications have been withdrawn.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.

No, push back on the gulley. There is no reg that says you need a gulley for a bin. For a single dwelling this is silly.

Sadly the bricklayers will find it very hard to work with and detail. I have never seen this installed correctly. You would be better off using Dritherm 32 batts, which are far easier to install. You will get a much better actual result.

The timber sleepers will rot out pretty quickly. Your tarmac looks fine. Go over the whole lot with 3 passes with a Wacker. Make sure the bedding layer is an even thickness.

If this is a sink cutout it will With bot circular and jigsaw you will get a much better finish if you cut it from underneath as it will help stop the laminate being torn off the chipboard. The sink edge hides a multitude of sins. Seal the exposed chipboard with some decent wood glue.

I hate plumbing. It is the cause of the vast majority of defects. I think we should bring back the outside WC. The hidden cistern is probably going to haunt me.

Well I have no idea how you would change these in a full tiled bathroom! I have 6 at home.

If they are too tight they will snag and you will lose edges, esp if you need to take a board down etc.

I have rewired places years ago where the vertical drops were in conduit. It made it much simpler than chasing in. For horizontal it was under floor.

I would use medium density blockwork. It is about 1400-1600kg/m3. Your marble will be about 2700kg/m3. With 2 layers of 20mm tiles and a bit of adhesive on 100mm blocks, you will be around 260kg/m2. If you did porcelain it could be 10mm. I would not want to tile onto aircrete blocks.

It may be worth connecting a rocker pipe immediately the drainage pipes exit the raft.

This is the way of things. Lots of back covering and padding. It is not in anyone's interest (apart from, maybe, yours) to get you to underinsure. Shop around as the likelihood of a successful claim for the full amount is probably vanishingly small.

It looks very difficult to reconfigure. Round here they often add an extra floor, remove most internal wall etc when doing a bungalow. Have you looked around to see if there are other properties that would suit you?