Hard to explain, the plastic with the metal thingy broke off, so the only thing holding it in one place was the boot. If the boot is opened, you could just remove the rear light by pulling it out from outside (without having to unscrew the internal plastic tightener)
Pics to follow, if I find myself a camera somewhere.
So, it looks like my weekend job will be trying to repair the stupid thing (hopefully superglue is good enough)
Ha ha, Ooops !
The same happened to me with the alloy hoops when I got them, although I purchased them direct from the manufacturer in France so when the fixing snapped whilst I was fitting it I panicked a little. If it wasn't for my deceipt and the fact that I know a guy out in France who was willing to lie for me (in French) I would never have been sent a replacement - and would have been stuck with one alloy and one rubberised hoop