I understand this thread is old but i thought i would shed some light.

I used a company called Ask Glass who cover the whole of the uk. They fit cat AND dog flaps into glass and wood and plasic doors. Badicly everything.

I used them to fit a cat flap into my patio door and it cost 170£ including the cat flap! I even got a free cat toy.

Would defiantly recommend them!

Ask Glass Link Here