If you look within the the petrol/ Derv cap, you should see a little pin that connects / inserts the flap to lock it, get someone to work the C.Locking to see if the pin is coming out, it may be broken or in need of a squirt of WD40.
Thank you TWR, the squirt of WD40 did the trick (and a lot of patience). The central locking doesn't do anything to the petrol flap on my Micra count-a-strong.