The fuel caps are renowned for doing this,the lock wears as the fumes from the petrol break down the grease in the lock.You can buy a rebuild kit from ford which will include a new barrel and new discs relevant to your key(which you will need to take)or if you don't want to tackle replacing the discs yourself take it to your local locksmith and he will rebuild it for you for about £20(just replace the discs).Quick fix anyway