Just been and looked at mine,, one of the last parts I havnt stripped off yet!
the plug appears to be in 2 parts, ie, top part pushes into bottom part to spread the retaining legs that hold it in the hole.. to remove, this could be tricky without damaging the plug.. feed a small screwdiver into the "top" cap, and gently prize upwards, the cap should come away, extracting the integral rivet pin at the same time.. shoot some WD40 into the gap to help things along, once the top is off, the bottom should easily flick out with a screwdriver. Too bloody cold in my garage to remove mine, so this is just an educated guess

Tom