I used the "Catflapman" after reading a recommendation on this website. He was very good and honest !! He told me that to fit the cat flap in a double glazes unit he would have to replace the whole unit and it would cost quite a lot - so he managed to fit it to the side of the window saving me quite a lot of money ! Give him a go - a few people recommended him on NVN.
http://www.catflapman.co.uk/