Was in C&R today for a fault code check and they reckoned it was bust, so I decided to have a look and see how it works. Each end cap is held on by a rivet and a torx screw. I undid the screw and drilled the rivet off the business end (the one shown here). Came off pretty easy. Had a look around with the cap off and tried to figure out how it all works. Seems incredibly complicated for something that really is just a retractable cover.
Couldn't find anything wrong with the mechanism but as soon as I touched the two prongs on the load cover they sprang out another 10mm or so. A bit more investigation and it looks like a lip has worn on the plastic mechanism which means they don't always spring back into position, they spring back on the business end side but this actually forces the other end out once the cap is back on. Took a bit of fiddling but I put it all back together and it's back in the car, although still not 100% securely.
Ebay here I come, I couldn't see anyway of getting into the spindle to fix the worn plastic.