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I was lucky to borrow a big dehumidifier in a similar position and it was a big help. Second the don't turn MVHR on yet.

next time i lay some flooring down I will glue them on as well as screw to help avoid any creaking

that is a lot of down lights! or is that just me…..

I had a very similar build up with the dpm as you have it and used mesh for slab strength and, as you say, something to attach UFH to. Not sure how easy it would be to clip the UFH pipes to EPS (might be easier with other types of insulation board). The pipes will be under a fair bit of tension especially at tight corners and EPS isn't the firmest/strongest material to fix to.

i found I got competent with a digger in a day. Obviously there is a difference between competence and excellence. Maybe it depends on how useful such a skill may be in the future. But it is fun to learn!

I don't know if wasps deter bats, but I do know that bats deter mosquitoes!

my biggest regret on my renovation is being a cheapskate and using some plastic guttering for the back of my house. We used cast iron on the front and cast iron lookalike elsewhere but i just can't stand plastic!

If it is only a few square feet you could always just do it dry. if you are going to try and expose the aggregates it might be easier to use pretty fine stones like in the example in the video. If you have normal ballast it will be a lot more work.

block is going to be better than stud work for sound insulation - the denser the blocks the better for sound.

Yes you can. Only question comes if someone complains and the council come and have a look and find you aren't within the rules. So make well sure it is within the rules!