The message
from Richard Conway contains these words:

One other thing. As you say the two walls are external I assume that
they form a corner externally. Is there a downpipe on this corner? We
had a similar problem and it turned out that the downpipe was not
properly aligned with the gutter and when it rained rainwater was
dripping down the side of the house.

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

--
Skipweasel
Pay no attention to that man behind the curtain.