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Until you can be bothered, clean and lubricate the hinge points with some 3in1 oil and a dab of grease on the point that locates into the top cap.
That would probably help it close easier and better (I'd recommend that as general maintenance before any fail)
If there's still a gap, pulling the top inwards with a suction cup whilst closing the window should work until the next time its opened.I'm just about to head off to job which I expect to be replacing a pair of hinges.
Edit, it was replacing hinges because the slider had snapped in two, so closing the window wasn't happening.
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