How would it look if you screwed a couple of wooden battens to the wall for the brackets to attach to, would that be sufficient for the top of the radiator to then clear the sill?
If it were a modest sill, that';s probably what I would do (I have in the past). However, to clear this one would require the rad to be something like 100-120 mm away from the wall - hence 'sticking out into the room'more than I would really want.
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