thats an extra that someones put on
hard to see cos you cant see the side whether its one of these anti-theft type ones
otherwise, it should unscrew
what about a small spanner as i see the head is hexagonal, probably work
 
It's after market - ebay probably. I would be fairly certain that it won't come off due to some sort of anti-theft screw. Have a look around it - I'd imagine it's probably a small allen-key to loosen before it unscrews.
I'd replace it with standard black ones. Noone will every nick them and they're easy to get off.
 
Easy easy, don't need go too ott, you can just screw the little thread thing out at the side with pliers, I just wondered if I could get the specific tool from anywhere.
 
its a tiny allen key (small back gromet on the side) and they are richbrook anti theft caps, you can get them in halfords, i had them but someone still nicked 2. Either buy a new set from halfords and swipe the allen key and take em back or try and find a really small allen key