I don't think that birds nesting in the eaves would really cause this, as they do choose places where there is no driving rain....I would check on the condition of the slates, on the two courses that exit into the gutter. Check for slipped slates, any gaps etc where water could get in, and see if there is a deterioration on the under felt which may have an influence on this. Maybe though, the damp has actually been cured and its just the plaster inside that doesn't like taking paint? In which case, you may like to consider 'One coat damp seal', Screwfix Direct number 38631, which covers damp marks, and gives a good primer for new paint. Only works if the water ingression has been stopped though!
John, May 2009