I have a 12" screen which i am considering modifying so i can fit a roof.
(Muzzy you were right  )

It looks as if i slide out the old glass and take it to a glass supplier that thay should be able to use my screen as a template and add a inch and a half to the bottem - my frame legs are long enough to cope with the increase.

Is it this simple or am i missing something - has anyone attempted this or should i just sell my screen and buy a bigger one?

Sounds OK, any Autoglass place or agricultural/plant glazer will cut you a piece of 'flatlam' for around £50. Get rubbers from Woolies trim supplies.