For the past few months I've simply had a 'cat gap', rather than a 'cat flap'.
That's because two fighting moggies blasted the flap off the thing and, as it was getting old and tatty anyway, I decided to replace the whole unit.
I made sure to buy the same make and (what appeared to be) the same model but the lady at the checkout warned me that Staywell had decided to make the hole required in the glass slightly bigger than it used to be.
I tried to fit the new flap but found that it it was just a millimetre or two too big. Since I couldn't just leave a hole with sharp edges in the glass, I then replaced the casing of the old unit, only to over-tighten a screw and put a great big crack across the glass. So I've currently got a sheet of glass in the bottom half of the door, held together with lots of gaffer tape, with just the outer case of a cat flap in it!