Hello Simon,
I believe it’s usually mounted pointing downwards (meaning the straight edge with 4 corners), but it works vice-versa, too.
Also, I’m pretty sure that it did not brake not due to wrong orientation, it’s just overused plastic.
(In other words - depending on bolt torque and clamp use frequency - it will break again sooner or later.) Fortunately it’s not an expensive part 
Kind regards,
Chris