I need your advise to the cause of mould detailed below.
I have 1950’s build 3 bedroom end of terrace house. We have black mould appearing in the back bedroom at the joint of ceiling and the outside wall. Photo uploaded. It only appear with the outside wall joint about half of the wall. It is 2 years old and have not spread. It was cleaned and returned. I have checked the roof tiles and they seems OK. The roof tiles are not broken and have never been replaced since we bought it in 1996. The guttering is clear of any debris. The guttering facia need painting or replacing as you can paint flaks appearing. The room is well ventilated by regular opening of windows. The room temperature is warm like a normal house, in fact sometime kids says it too hot and they open the window. The loft have several inches of loft insulation. The loft is not jam packed with stuff and is reasonably clear. The loft water tank is fitted on top of this room with no leaks.
Can someone please advise me the cause and cure.