Have building control asked for acoustic tests? What does the lease say about carpets and soft floor covering.
Sounds like airborne sound insulation should be o.k, assuming there isn't a significant flanking path.
To control impact noise I would suggest JCW acoustic deck or similar
Do you really need three phase? My quote between single phase and three phase wasn't that different, so went three phase though don't need it.
Have they quoted for the trench work from the road to the house as iirc its quite a distance.
I am getting flash backs to another eye watering tender that got posted for a refurb in London and the chap seemed to get his pant's pulled down by his 'mate'. I can't find the thread now.
found them
https://forum.buildhub.org.uk/topic/32386-what-are-your-thoughts-on-this-project-estimate/
https://forum.buildhub.org.uk/topic/37290-dispute-around-construction-costs-also-posted-in-the-finance-forum-wasnt-sure-which-was-best/