Thanks ajn. I am amazed at the lack of interest in cat flaps on this forum. Clearly some effort should be expended to raise awareness of this important subject.
Kezo, their seem to be no direct comparisons but I looked at the two most popular ones in the local pet shop and bought the one which appeared to be best quality which was also the cheapest, which if anyone cares is the Closer Pets / Catmate one. Now in the process of fitting it through the conservatory wall.