Several of my uPVC casement windows have a draught due to not fitting tightly against the seals when closed. The gap between the casement and the seal is not uniform along the height of the window. One of the poorly-fitting windows has a plastic bracket in the hinge mechanism that is clearly broken, but in the others this is sound, so it doesn't seem to be the crucial part. I can't see any parts in the hinge mechanism that are obviously intended to facilitate adjustment.
Bottom hinge of kitchen window (intact bracket shown):
Top hinge of kitchen window (mirror image of bottom):
Closed; showing the gap between the casement and the seal:
How can I adjust these windows to form a tight seal?