Hi All

I am in the process of selling a 1983 Sierra Ghia Estate and want to get the fuel cap off.
I have what I thought were the correct keys (boot and door/ignition) but none of these unlock the fuel cap. The key turns about quarter of a turn but the cap won't budge.
Am I missing something or are the keys I've got not correct?
It isn't my car - I've just been tasked with selling it, so I've never needed to add fuel.

Any help welcomed.
Thanks