we had this done a couple of years ago.
was just over 200 quid fitted.

however, if you are willing to do the work yourself (ie all the measuring and fitting etc) there are glass companies out there who are happy to cut the glass to your size. was quoted £95 for this option but opted for more expensive in the end as i wasnt sure of measurements.

hope this helps someone.

ps. if you ever need to change your flap try and find one that is same size as previous hole to save on expence.