On Wednesday, 15 May 2019 17:32:57 UTC+1, George Miles wrote:
there was cold air the other side of the roofing felt which is why condensation happened there, not anywhere else.

If it was as straightforward as that I would have expected the overlaps to have suffered similarly as there'd be very little, if any, temperature difference there.

Some have questioned whether it really is mould and I am leaning towards that too; not due to wishful thinking but rather that the basis of the initial hypothesis is not looking strong.

I'll given the felt a vacuum, try some of the hydrogen peroxide that arrived today, and see how things go from there. Finger's crossed I won't see it again or, if I do, it'll be in another few years time as appears to have been the case here which should point to it being something more benign than a ventilation/mould issue.