The car key is stuck in the petrol cap (which is broken)what can I do ?

My car key is stuck in the petrol cap and the cap is broken - when I turn the key it won't come out (it just rotates )
I've tried using WD40 to no avail ,so I'm going to have to get the cap off some how .(the cap is in the car )
I hopefully won't break the key ,but right now I've got no idea
what to do ?
It's a ford fiesta .