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What have you agreed with the Architect? Some contracts are limited to getting Building Control Approval, others include construction drawings. These are sometimes the same drawings but not always. The minimum required by Building Control is less than required for construction drawings. On our house we had to use denser blocks in a few key areas like pillars supporting steel beams and between two windows where loads were higher than normal.

Most of us just yell at the kids... Turn the f... light off.

It could be the lead valley was designed/installed incorrectly so perhaps don't just copy what you have with new. Why did it split? Were the strips of lead too long (thermal expansion)? No expert but I know lead work is a bit of a skill/art.

This bloke seems to know what he's doing. See if he has a vid on valleys.. https://youtu.be/srpMbLCTnMg?si=hFricBFGC_FSeikN

Both. Building Control normally want to see drainage plans and may want to inspect before trenches are back filled. The utility will also want to approve a connection to it. Think I would get some drawings done and approved by the utility co. Think it's unlikely or less likely BC will object if the utility co is happy.

The more insulated the UFH is by carpet the higher the UFH temperature has to be to push the heat required into the room. Ultimately that can affect the efficiency of the system. But if your house is very well insulated that might not be a problem. Does it even need heating upstairs. We have some rooms with 21mm wood floors and have had to raise the UFH temperature to 50C when it's very cold outside. But our house isn't super insulated.

For small qty I'd use one of those big blue flexible buckets and a plasterers whisk in a drill.

They recommend you keep the total TOG value to around 2.0 I think. You might get away with a bit more but how much is hard to say. There are special low TOG underlays that allow more to be allocated to the carpet. Note that some carpets are available in two versions foam or Hessian backed. The small print in some contracts allows the shop to choose which they supply. Hessian has lower TOG but a harder feel. Go visit some carpet shops and ask about TOG values. If you get blank looks ask for another staff member or go elsewhere. Try out samples to see if the carpet and underlay with a low TOG are ok underfoot. Then you can decide if you need thicker.

I've used VP400 on a project and its quite robust. It was exposed for a few months and survived ok.

Use a vapour permeable "felt"/membrane before battens and I think you are ok. Check the makers instructions and show to Building Control before you install. Remember the membrane must still drape so a 25mm unventilated void maybe required. This is to stop water pooling above the battens.

Would it be possible to jack up the joists and put packing (or just DPC) under the ends?

Does sound odd. On a previous house I found a set up that had one side of a light connected to live. The other side was connected to a switch that supplied the light with either live or neutral. So when the light was off BOTH sides of the lamp were live.

Try the bulb in another fitting? Have you checked the configuration switch settings? I would note the default settings and then try the last example (works night and day). Try temporarily hooking up a simple pendant to check the house wiring.

If you buy that type get a few spares. If they are any good they do last a fair time but better safe than sorry.

I rented a "power scythe" to deal with the vegetation but it sounds like you might need a digger with a blade on the front. Got any friendly local farmers? They could plough it up and reseed it for you. Otherwise get quotes from grass or landscaping companies.