On Sunday, 12 May 2019 21:51:51 UTC+1, Vir Campestris wrote:

I wouldn't bother. It'll die when conditions are no longer suitable.

I've found that vacuuming it makes it far less noticeable, so perhaps that might be suggesting it is not all mould (i.e. also general dirt). That or I've just blown mould spores out from the vacuum extract!

The bigger question is how is the damp getting into the loft. I don't
suppose you have a bathroom extractor that vents into the loft instead
of outside?

All extractor's are properly vented, however in typical new-build fashion the house has dot-and-dabbed plasterboard walls but in something of a 'plasterboard tent' form which almost certainly creates some airways all the way from the the ground floor upto the loft. With the ground floor currently stripped back bare to build an open-plan extension it wouldn't surprise me if warm, moist, air is rising up behind the plasterboards all the way to the loft. I'll be foaming up around the ground floor perimeter when I get a chance so if that's is a contributory factor that should help.

All that said, I've just dug some loft photos out from last year and they do show something similar so I am now thinking it might not be as 'new' as I thought, and perhaps more also not be as much mould as I thought either.

Might just keep an eye on it, and as mentioned, improve our ventilation prior to the MVHR going in.