OK! Well I've spoken to my Chinese friend, Sum Ting Wong, and he says if graphite don't fix it (as DP mentioned), its time for a replacement fuel Cap!
Doesn't the whole unit come out, after you've removed the Hex bolts around the outside of the 'circle' then you pop in the replacement part.
I'd even go as far as saying, I think the actual lock itself can be removed from the underside, with screws, so you could swap the lock over with your current one, and not have to use ANOTHER Key for the fuel cap (different from your ignition).
Depends if its your lock that's had it, or the complete mechanism.