The tool to use can be a 2.5mm long arm allen key which fits the hole quite nicely and the long arm helps with leverage.
The blown lamps (notice the discolouration):
Two new lamps to be fitted:
C3 Number Plate Lamps available here on Amazon and they are also found on eBay when you click here.
To remove the plastic cover, insert a 2.5mm long arm allen key (or similar) into the hole, as shown in the photo. It only goes in a few millimetres. (The cover has been removed from the car for a clearer picture).
With the allen key in the Right hand hole, angle the allen key slightly to the Left and pull slightly. You may need to do the same to the Left hole to fully remove the cover. The lamps just pull out and replacements just push in

To refit the light covers push them into position then insert the 2.5mm long arm allen key into each hole in turn giving the allen key a push to re-engage the clips.
If you are unlucky enough to break the plastic cover, or find its cracked then its an easy job to get some new covers and lamp holders sent to you - click here to find them listed for delivery.