| Do you have good access to it?
If you could cut a small groove in the outer edge, you may be able to tap it with a screwdriver/chisel and (small) hammer to get it turning.
Dunno if that makes sense.
I've managed also to get a screwdriver (another bodge) that is like half a millimetre too big for the hex head... tap it in so it's snug and then try turning that... even better if you have a screwdriver with a spanner attachment thingy-me-bob
I'll try to think up a 3rd bodge, should all else fail. | 
domski
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