Mr Punter

If you have a 50mm layer of rockwool compressed under the sheets it helps a lot, but you would need to take the sheets off and refit.

Where we are, if you don't have the right ducting the DNO (UKPN) won't run the cable. They will tell you what they need. Just re read and I see this is your side of the meter, so 100mm rigid duct will be OK.

The bricks you have shown do not look standard size of 215 x 100 x 65 they look longer or not as high. There are quite a few foreign bricks non-standard sizes. The proper lime mortar looks good. I would be tempted to get local clay stock bricks as anything imported can have a fair lead time and delivery charges.

Agree with @BotusBuild. Aco drain will work well here. Hopefully there is somewhere you could connect it to at the end.

Tell them you can set the test up and they can come and witness it. I would not want to spend £400 just because someone is being a numpty. Part of what you pay for is inspections.

If you are mixing in a gorilla tub, let it go off a bit, then empty the bucket and smash the stuff into smaller bits.

DON'T PAY ANY MORE! Write to them explaining that the work is not satisfactory and you will be withholding all further payments. Attach the photos. Reserve the right to claim all expense to rectify.

The work is so bad that I would not offer them the chance to return and make good. Your new firm will need to strip the whole thing and start again. Make sure you see some completed jobs and have a chat to customers.

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.