Remove the grey gasket and you should see either 2 screw heads or 2 small bolt heads (both types of fastener are used by MMC) if you undo these the lamp removes and you can see the 2 matching lugs cast into the plastic plinth. If these have been removed and as you suspect the lamp was glued on its probably because the plastic thread strips if over tightened (or splits) and they/someone have siliconed the lamp body on(not the first I have seen). The brass plugs as mentioned above are held in place by insulation tape which is crazy as they match up with a void in the plinths so once wet the tape falls off. I made up some 3mm ally backing plates for all my lamps to 'sandwhich' behind the lamps and hold the brass slugs in place....nice little winter job ;-).

BR Colin
Who used to be a Spanner Juggler