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Painting it will be fine if just standard emulsion I have never had an issue with tiling onto painted walls in a kitchen. Also if you ever decide to change the tiles in the future, there is less chance you will destroy the walls taking the tiles off

Request them to remove them and use them as cuts if possible. They're shouldn't be blocks with large sections missing they should be whole and in good condition

Don't rule out rockwool nyrock frame slab, I have used it several times now and an excellent product

I have installed a large amount, they are both very good . My personal opinion is I prefer Hardie especially the VL plank

If in doubt especially for a sole plate, I would use pressure treated timber

Difficult to tell from that picture, could just be discoloured from age or the sun, but could also be vac vac treated. I would say it's not pressure treated personally . Trim a tiny bit off the end and see if there is impregnation marks

As said D4 Spax wirox flooring screw

As said Hep is best and in my opinion a superior fitting to JG speedfit and polyplumb. Keep the pipe clean and scratch free, make sure ALL the pipe is cut using a proper cutter. Use the barrier pipe with the Hep stainless steel insert in every fitting . The majority of all new builds use this system now and if installed currently it is trouble free . One downside I have seen several times now is rodent attack

That's sorted then . Personally I would try and blend it in with your neighbours, a better job long term

As you say a bonding gutter can be used. Or speak to your neighbour and take a couple of rows of slate off their roof , felt abd batten it overlapping into the original roof and tie it all in properly. Just be aware the corrugated sheets could possibly be asbestos. A simple sample test will confirm this either way for about £30/35

No problem, they may well have security clips on the outside depending on the age of the windows , some deduct a few mm overall to allow a slight tolerance for this . Personally I have never had an issue.

If measuring from the inside on external beaded upvc windows , you need to add 15mm either side / top and bottom for the correct measurement: ie 30mm overall on the width and height of the DG unit off the internal bead, that's measuring off the plastic, not from the rubber gasket. Doing this will give you the same measurement if you were measuring from outside and taking 5mm off either side , so 10mm overall Ordering formula = Width,Height, Thickness, clear or obscure glass , then if toughened is required or just float .

STS, never had a problem on TF properties

Marsh industries, they will answer any questions and supply

A 32mm core drill would give you the neatest outcome