I am refurbing a house that already had UPVC windows. for some reason one of the tilt and turn windows allows water in, at bottom corner. i was told to try drilling the frame from outside to see if drain is blocked and frame was full of water, but found it was empty. there is a inner drain hole just inside the frame next to the outer rubber. the water poss comes out of that then pass the inner seal onto the sill, so how do i stop this?