If it's a door with 2 separate pieces of glass, it might work out cheaper if you replace the bottom pane of glass with plastic - that way you can simply cut out the desired hole and fit the cat flap yourself :)
Thanks for the answers. There's no brickwork, the entire conservatory is glass and each pane goes from top to bottom so if I had to replace the panes I'd have to replace to quite large pieces og glass.