Best advice would be to contact a company specializing in double glazing repair. They will be able to supply and fit a new panel in either uPVC or toughened glass if you prefer. I do this all the time, and it is easy if you know how and have the right tools, but there are too many subtle variations in how the panels are held in place to give you advice on how to do it with your particular door. Would come and give you a price myself but you are a bit too far away from Hampshire!
Peter Gee STEAMY WINDOWS, February 2010