Fuel Cap

Have just purchased a 2002 Focus.

Went to fill with petrol this morning but couldn't taken the cap off.

I can open the initial compartment on outside of the car with the key you use to start the ignition.

However, then I get to the cap where I have to put a different key in.

It takes the key ok and unlocks. However, its when I begin to unscrew the cap, its just won't unscrew and come off in my hand, its just spins round and round continuously.

Now I don't want to use brute force to pull it off incase I cause permanent damage.

I think its obviously been put on too hard by whoever had it last !

Any help or advice grately appreciated, especially considering i'm on empty !!

Cheers.
Matt