We had the same problem on our 45e. The glass is fixed to the timber with some sort of adhesive sealant. It has just been applied in a “squiggle” rather than all over. We cut through it using a windscreen replacement wire saw (check ebay). It isn’t easy!
Just buy a replacement mirror from a glazing merchant but specify a ground edge. Before you glue the replacement back on, give the back and edges of the new mirror a couple of coats of two pack epoxy varnish. If you seal the water out the new mirror will last almost indefinitely.