When you eventually get it off, do what I did.

I bought a complete lock replacement kit from ford for not that much (less than £50) which provided me with a complete set - Ignition, two doors, fuel cap and boot.

I did mine, and it was like a new car! I was having similar issues with my cap as well

I konw this wont help you get the thing off, but when you do, it will help you solve it in the long term

JJ