Hi .
There is a shallow round annulus, about 3" diameter, that the mirror backing moulding locates and clips into. If you can, move the mirror so that the outer vertical edge protrudes from body. Then pass a flat plastic scraper, or similar behind (being careful not to damage heating element connectors) and gently lever/snap mirror away from the head. Dont use a screwdriver .. cos that can shag it ; )
Easy!
Peace ... Bags.