Ceiling boarded - apparently the upstairs subfloor is only 5mm plywood - which explains why it's so bouncy & noisy.
Annoyingly there's nothing I can do to reinforce from underneath whilst the ceilings down; we need to empty both bedrooms, lift the carpet & underlay and completely reboard with minimum 9mm plywood (I think we'll go for 12mm, as it's not a huge difference financially).
That's a job that'll have to wait until after Xmas, as finances are really tight between now and then.
In the meantime, plasterers boarded the ceiling today, ready to skim tomorrow & Friday.
Picked up some paint samples to go over the boring & tired grey. We're going to put acoustic panels on media wall & opposite (vertical walnut on black felt), so we're after something with a bit of warmth to contrast: