Mike makes a good point about the pitch of the roof. Your assassin might move along the ridge, but unless it's the sort with windows set into the roof -- which would be unlikely in medieval times, I would think -- they're going to have a devil of a time keeping from sliding down the sides. Especially in a wet climate. I don't know about medieval Scotland, but roofs in our area are thatched with reeds which shoot the rain off and it would be hard to get much traction.
So to answer your original question -- yes, I think your character might hear someone moving along the roof ridge.
I just asked my local expert, who added that A) It would depend on their footwear and B) even if their footsteps couldn't be heard, they might well loosen bits of thatch as they moved, which could be heard falling.