OK ,that rules out the possibility of travelling damp. I notice there is a big difference in the two pics with regard to the density of the mould. I assume it is laying on the surface and you have easily removed some if it , by chance or on purpose. Anyway ,if you can remove it and the plywood below is sound its obviously not inherent or penetrating from below. Is this so ?
Hi Terry,
Yes, in checking how bad it was, a screwdriver and stanley knife were used in probing the wood and flaking pieces off, I also used a brush to collect up the pieces of plasterboard I cut out so that area has been rubbed a lot.
I really wanted to get that whole section up to check the underside of the boarding but frustratingly, where there used to be an inspection hatch cut into the old floor, this was removed when the new boarding went down and its now a large, complete sheet.