elite

30Mbps should be good enough for video calls, but of course depends on how you're connecting and building construction etc. I know I would prefer a wired connection. RE openreach, make sure they understand that it isn't a new site and there is presumably a pair on the pole that served the bungalow (unless someone else is now using them of course). Have you checked altnets? Giganet and CityFibre might be in your neck of the woods

Can you do warm roof but take back the house roof to give you the height? You need careful detail around the transition to ensure ventilation into the cold (I assume) house roof. An wider external pic might help others give more suggestions

Interesting, unusal for a switch to cause such a fault, but I hadn't realised it was an attic switch which I guess is more "exposed"

You need to keep splitting the circuit until you narrow down the cable with the fault. This will be a lot easier with a tester that can do insulation resistance testing. You can pick up used testers on ebay, but probably cheaper / easier to get a spark in

How is the pointing? The flashing doesn't look the tidiest. I had similar but was able to remove the stack, and the roofer said it was so bad he didn't need tools to remove the bricks 😬 Has a good roofer had a look at it?

Is the floor "bouncy" or is it just squeeking boards? If just the later, as @joe90 says, just get some scews driven in to pull everything tight

https://www.ironmongerydirect.co.uk/product/altro-disabled-bathroom-turn-&-release-suit-8mm-spindle-satin-stainless-steel-234592 ?

How many cores? How is it installed? This is a useful calculator: https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

Not a plumber Is there air in the system? Does the pump have somewhere to pump from? e.g. are radiator valves / TRVs open?

I was going to say I'd want the control room bigger if it was for recording. My knowledge is more on a recording build, but essentially bass will be your biggest issue, so lots of mass and decouple everything where possible. You will also want to kill reflections etc especially if you're loud, so soft furnishings will help a lot with this, if you're not able to build anything in at construction. Also bass traps in unused corners (wall ceiling junction is a corner too 😉 )

As above, you need mass your best option is probably MLV PB sandwich Is the space primarily for rehearsals or recording?

Pay particular attention to detailing on doors / windows, these will be your biggest problem. Is that a french door to the left of control room? It will probably be more leaky than others. Consider making stud walls double & decoupled Check your room ratios aren't going to cause acoustic issues I suspect you'll want the room pretty "dead" at that sort of size, so incorporate absorbers / bass traps in the design early on A great resource: https://johnlsayersarchive.com/ Keep us updated on your progress

Looks like a warm roof from the exterior pic. Ventilation through the wall if possible to avoid penetrations to the roof

Something like this?

Presumably you want the reatining wall to be stabalised, so I would be helpful as far as possible without it negatively impacting you significantly. What were the nature of the works you undertook to dig out? Was anyone involved to assess the impact on the wall? It is good that you have photos of the wall before works were undertaken