Roof shouldn't really need felt to be dry, my parents house and
several friends have dry non felted roofs, what is it tile/slate ?

Tile. Maybe it's possible that one of the tiles has also slipped then,
although I can't see anything wrong from the outside. I'll have to take a
bit more of a look. It does only tend to leak at certain times, presumably
when the wind is blowing in the "wrong" direction.

Thanks,
A.