Mr Punter

Umm it was actually @BTC Builder who was the OP and had the hold up!

Well you could gag and blindfold them I suppose...

You may as well get someone to supply and fit, as you need to meet electrical and Fgas regs. They are probably busy now. Have a ring round in a week or two.

Well that is good news and pleasing that it didn't result in any lasting umbrage.

With the roof tile vent you don't need flashings but you need the right vent for your tiles. They have a flexi connector.

These are often standard drawer boxes from Blum etc. Have you ended up with things that don't fit? Did the supplier have a showroom or any information on the drawer depths?

Ask the SE. It is their job to specify correctly.

Crushed concrete / brick will be OK but not any old crap with plaster, wood, rubbish mixed in.

A friend was doing and agricultural shed and he went for timber frame 8 x 2s as you only need to nudge blockwork with a telehandler and it collapses whereas timber frame bounces back if it is constructed correctly.

The 22mm is annoying but is only 1mm per metre. With the sub structure stuff it will be possible to rectify but I would point out that I expect it built as per drawings. Better to let him know as he progresses than after he has finished. Check the drawings each day and go through with him in the morning where you think he might mess up.

I am hot on avoiding on high risk / severe consequence incidents, so falls from height, buried in a trench, brick on your head, crushed by materials / machine etc.

Timber frame stick built with mineral wool insulation and 18mm OSB internal sheathing. External cladding of your choice. Strong, simple, lightweight and cost effective. Pitched roof would be my preference.

We have used the Kytun Ali trims on natural slate. No issues with slate / slate and a half.

I have not used this one but have been able to get an email address to send Build Zone documents in the past, as at the time their portal had file size limits. Call them and ask what they suggest.

Have you been in touch with the tilers, or was this DIY? I have not used fix-a-floor but there should not be problems with the tiles over such a large area. If it were me I would do a small area and monitor before taking on the whole floor.