wozza

You can buy downlight hole repair rings - search amazon / Ebay

Can you not fit the tray before building the wall?

I believe that the "Rain" sensor is built into the part with the solar panel - can you easily get to the window to test them. If I gently tap (simulate rain drops) the top part of my windows where the solar panel is they will close.

Could the bath go on the toilet wall? Then toilet and sink on the right wall - this would give window access and allow a shower above the bath.

Also consider Shelly. They now do wall switches with a plate to house the WIFI switch or dimmer - I have four Shelly Dimmer 2 and they are working well - lights are controlled by a standard wall switch (Shelly switches were not available until recently), the app or voice assistant.

They are designed to snap off with a sharp blow from the side along with the grout line (do not try against the grout line) - you should be able to see the break point. YouTube has lots of videos.

The levelling spacers are typically removed by hitting them side on with a mallet (or kicking them if on the floor)

Our extension roof (vaulted) had 100mm PIR between the joists and 75mm under. I created a service void using the MF Ceiling system (Metal frame) its really easy and something you can do yourself. We boarded over the top then used Aurora angled downlights (that can split in half to reduce the depth).

We had ours from Benchmarx, cant fault it.

You should be able to install a speed test app on your TV so can check speed easily.

I have a lot of LED downlights - I use Shelly Wifi Dimmers and a Varilight Supla Wifi Dimmer. I can control my lights with Alexa, the apps or via the simple wall switches.

We had Cemfloor. We fitted thin blue foam strips (UFH Stuff) across all doorways - no cracks apart from where it covered the foam. We were told by the contractor that we may get a cracks at two certain points (he had seen some similar floors crack and others not) and that it was up to us if we fitted foam, we didn`t fit foam and it cracked. He said it was normal and not to worry.

I don't know about the Neff hide and slide ovens, but when you open the door on our Siemens ovens the fan stops.

Using 110mm soil pipe with that extractor will be a bodge, using flexible duct of any size is also a bad idea (noisy due to turbulence from the irregular shape) IIRC my last extractor stated that the duct should be 150mm, but would allow 125mm. Have you considered using oval / rectangular duct - the high end stuff has vanes in the bends to reduce turbulence (turbulence reduces airflow and creates noise)

My in laws have a Neff hide and slide - they like it. SWMBO tried it several times, and also went to a cooking demonstration etc at a local kitchen showroom but decided to have normal Siemens ovens when we did the Kitchen a few months ago.

Condensate pipe must be plastic due as the condensate can be corrosive. I have a Copper mushroom on my boiler blow off - very flush looks tidy - most fitters use a couple of elbows to return the pipe outlet to face the wall so any hot / pressurized water is not likely to hurt anyone.

It was sometime ago when I got mine - got them from here: Yale Platinum BS TS007 3 Star Euro Profile Cylinder - Int 35/35 Ext Nickel (lockandkey.co.uk)

The fitting that the 21.5mm pipe goes into just doesn't look right - its hard to tell from the photos but it doesn't look like a solvent weld fitting, also the 21.5mm pipe also appears to be at quite an angle to the fitting which will cause problems - what is the 21.5mm from? a boiler condensate? Can the 21.5mm pipe be pulled square?

The first fixing you tried are for solid concrete / brick - the metal collar is an interference fit / tight fit when inserting - then when tightening the conical end forces it to spread in the hole. I have recently used some chemical fixings and will reach for them most times now.

Can you get a better photo and measure the size of the two pipes - looks to me like the wrong fitting has been used.

Before using anything harsh, try Windolene (pink cream stuff) or as you have said a cream type car glass polish.

Radiators were typically placed under windows as single glazed windows were the coldest part of the room and the cold air from the window would help the warm air from the rad to circulate around the room better.

This video shows the issue with the thread leaking: How to fix wash basin waste leak for good How to seal waste. - YouTube If the new plumber did not seal the thread then do not keep his number.

Its hard to see much from your video - the leak wont be where its dripping - it will be somewhere else and tracking to the point where it drips. Is that a shed load of silicone bodged on the waste thread? Wastes can leak water (around) through the thread around the nut - its a common problem - I would be checking: if there is a seal between the waste nut and the sink and then Is there something to seal the thread on the waste where the nut currently is?

If you are having a Quooker then you need a waste pipe - as PeterW has said they have a pressure outlet that is plumbed into a drain pipe. I doubt that a Sink trap connected to a pipe through a sound proof wall would be a concern or allow enough noise transfer to warrant concern - is the wall sound proofed for a specific reason and how is it sound proofed?