I've managed to get it open - by pushing upwards towards the rear of the glass. This slid it open, however initially it was sitting on the roof - i've since loosed up the little mechanism on either side, so that it sits up a bit now.
However when I go to close - same issue - gets stuck at that point.
Is there any sensors or something around there, that I've inadvertently covered up with grime ? also the little tab has not reappared

heres a pic of the rail now. tab is in the inner yellow bit.
