Mr Punter

It is a bit tricky as traditionally t&g floorboards were nailed down and there was expansion available between each board, which could be 5mm. If they are glued together the 5mm no longer applies and has to be multiplied by the number of boards which would be too much over 4.5m. Maybe the battens min 45mm will be better.

I doubt glue down will be good, so battens fixed to the floor with concrete screws. You could run cables in steel conduit if you need to. Floating floor could also work but you would need perimeter expansion allowance and tongue securely glued to groove.

If they are inside the shed I think you will be OK. You can always lag them a bit. In the unlikely event that it does get very cold, turn off the water and go to the launderette.

For clothes drying, a dehumidifier in a small, insulated room works really well.

They look very useful. I have used straight ones with a 45 either end, but that meant extra fittings, more cost and more space.

A few small points: Maybe look at a fire resistant membrane on the walls. The box gutter may fail at some stage and possibly rot out the walls. The Kingspan phenolic insulation is good (better insulation and fire rating) but expensive.

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.