Anybody have experience with a removing a stuck petrol cap? 

I've tried using the key in the fuel cap, twisting the key, then the cap, this only unlocks the mechanism but allows for the cap to spin freely without coming loose- I suspect the locking mechanism to be broken but perhaps I'm missing an additional lock release somewhere inside the cabin. Any advice would be appreciated

This is on my new 1999 Felicia, pretty mint condition aside from a few crusty bits, 85,000km.

There is no additional lock release inside the cabin. No Felicia had that.

The cap has a broken lock. You might get lucky and by jiggling the key while twisting the cap will free it. You'll need a new fuel cap with a new key just for it.

Wd 40 is your friend