Lec TF5517W Bulb Cover Removal

I need to replace the thermostat on a Lec TF5517W. In order to do this the bulb cover has to be removed in order to gain access to a screw. The problem is I can’t get it to move. The manual just says push it forwards and there is a small illustration. Does anybody have any experience of removing one of these? All that’s happening at the moment is I am damaging the plastic.