Thanks for the detailed write up and pictures, made this a very quick and easy job that spared me from destroying my locked fuel flap in frustration.
Similar to mellor above my issue turned out to be a loose push fit pin going into the motor. Bit of a flimsy design but at least a simple fix. I found a good belt and braces fix for this courtesy of BestGear which I'll undertake should the problem occur again:
https://www.volvoforums.org.uk/showthread.php?t=194334