It sounds very similar to GF's 02 VW Polo, where the whole cap moves if you just insert the key and turn it. You need to hold the cap in your left hand to prevent it moving, then insert and turn the key to the left with your right hand. This unlocks the cap. Then unscrew it by turning cap to the left.
When locking it you do the opposite - close the cap by turning it to the right. Then hold the cap with your left hand and turn the key to the right with your right hand.
Dont feel silly, spent quite a while at that bloody Polo myself the first time I filled it up!