the original poster has said that theres lots of steam from laundry and boiling which is maybe getting up[ behind plasterboard - it will find a way up there as theres a pressure difference!

And he's working on a second hand MVHR

So I'd make the holes for MVHR and get it working without the complicated heat recovery stuff and see if that solves the condensation, if not put in more air vents.

(may have to wait till winter to fully test it)

I've insulated above my attic floor with wool and kingspan so i can have lots of draughts in the attic.

a dehumidifier in the attic wouldnt be a long term solution,
anyhow with this week of hot weather it just needs lots of draughts and it will dry out.

a humidity meter with hourly readings and a graph?

https://www.designingbuildings.co.uk/wiki/Dew_point


https://www.google.com/search?q=insulation+dewpoint

george