Guy King wrote :

Or water running down the inside of the roof timbers from a hole. Seen
that several times recently

There is no downpipe on that corner. The guttering at the top end
there. I will check next time it rains hard to see if it is leaking or
overflowing due to blockage etc.

As for water running down the inside of the roof timbers, I can go into
the loft quite easily. What should I be looking for. Do I need to
wait for heavy rain, or is there some damage that I can look for. Note
the mould is towards the floor not near the ceiling - would this be in
line with this type of fault?

Thanks

Bhupesh