Sorry Mags - it can't be done. Trying to put in a flap would break the seal, and assuming the glass has been toughened (as it should have been for a double glazed door), it can't be cut. You'll either have to get a new panel made with the cat flap in place(£££) or find somewhere else to put the flap.
Danzigger is right, I'm afraid, we had a devil of a job finding someone to do this and it did involve new panels and a bit of cash. We used these fellows http://www.24hourexpressglaze.co.uk/ who did a good job but there weren't a lot of other options, to be honest.