The double glazed window on the first floor of the gable end of my house leaks on the inside if the rain is blowing from the right direction. The water comes in from the join of the top of the window frame and the underside of the reveal where the lintel is. I have applied loads of mastic outside around the frame. which has made no difference. The bricks and pointing of the wall are in good condition i.e. no cracks, holes or the like. Can anyone suggest a way to cure this please. Thanks for suggestions in advance.