On Tuesday, 14 May 2019 15:42:12 UTC+1, Andrew wrote:

Did the builders use foil-backed plasterboard for the
upstairs ceilings (they should have).

No, just paper-backed plasterboard and a skim coat.

Do you have downlighters in any of the upstairs rooms,
notably the bathroom ?. These will allow warm, moist
air to leak into the loft.

The only penetrations are lighting cables and an en suite extractor; all seemingly well sealed.

Ditto a badly fitted loft access hatch.

That too is pretty good - it's an insulated affair with a well fitting compressible rubber seal.

Then there is also the possibiluity that warm moist air
is getting into the cavity and the water vapour molecules
migrate up the cavity, into the soffitt space and then
back inside your loft.

Could well be; unfortunately I can't see if the cavities have been closed at the top due to the design of the roof.

I'll give the felt a good vacuum, and might test some hydrogren peroxide to see if that cleans it up. If I can get it back to a 'mould (or whatever it is) free' state then I can keep a proper an on it, perhaps with humidity monitoring as George suggested along with visible observations of the felt.