Thanks everyone, just couldnt get my head round why its always on the bottom of the fram almost like the lip of the pvc at the bottom of the glass, sometimes it looks like it does it the pic sometimes its more faint and almost like just black grains gathering qt the bottom, 9/10 it comes off the bottom windoss with just a cloth and if it leaves a stain i let the sun bleach it, as im always quite reluctant to get any extra cleaner out if im not being paid extra