Kitchen sink is just off to the left and the drain vent is pretty much straight up from the stained area, makes me think the shrouding around where the vent exits though the roof is damaged and I am getting some kind of ice forcing its way in and running down around the vent, I can't see ABS failing all of a sudden. Dug around in between the joists in the basement and its wet it the one space only the one the drain comes down through. So I think I can rule out wicking.
Think its time to call a roofer.
thanks people