I'm starting to wonder that myself Brian. Certainly I couldn't see any 'fur' that I've seen accompanying black mould I've seen in other houses / on the web, indeed there doesn't look to be any 3D properties to what I've got as far as I can tell.

On Monday, 13 May 2019 07:23:59 UTC+1, Brian Gaff wrote:
Yes I was wondering about that and indeed whether a bit of condensation from
the causes mentioned would be enough to create it.