Mr Punter

PIR inside, glass wool outside.

If they are being plastered that should suffice.

Three storeys up on a ladder with a bucket and trowel v £50-£100 for a pro with a high reach vacuum to do it safely from the ground.

Rockwool is about half as thermally efficient as PIR, so would you need twice the thickness for the same insulation. Insulated plasterboard has either PIR or EPS. The EPS is between rockwool and PIR for performance. If you have space you can build a new timber wall and insulate it.

Yes a bit odd. It is fire and acoustic rated. Maybe the octagon made it a bit stronger?

I have only seen them used for external render. The OP is doing tape and fill plasterboard.

It is good to have a grille between the gutter and the surface water sewer to trap leaves etc. No need for a trapped gulley. Rodding is never likely to be needed although you may get roots invading and it is best to have some access to deal with this.

I have just done a small bit with the metal reinforced paper tape on some boxing. It was not too bad. If I was doing it on price we would be hungry!

I have had these fitted in some flats. They are quite clever. If you wire in a switched live they also look at how long the light is on to work out the overrun needed. Cheap too.

I have a 100A single phase supply and use it for the house and to supply a twin EV charge point that will supply 2 sockets up to 7.4kW each. Some of the plug in hybrids have a very small battery / electric range so they may not be optimised for quicker home charging.

So internally insulated block cavity wall. Are the internal walls mostly timber stud (other than the brick one)? Do they go up after the insulation / battens? BTW if you want someone to see you have mentioned them, type "@" and then pick their name from the drop down list @Gary Martin.

Notching and noggins fitted tightly between the joists is quick and easy.

Yes it looks fine to do. There may not even be a lintel above the door, but better to put a new one in as you are altering the opening and the making good will be easier.

It is perforated gas duct.

I would hoover out as much crap as I could then slide some pipe insulation down as far as it will go. That will stop the condensation. A bit naughty using gas ducting but it could be worse.