Ok, have taken some pics. The flap itself measures 14.5cm wide by 14cm high. The unit is 21cm c 21cm.
First pic shows the "tunnel" it comes with (you can't trim this as the scanner is inside the tunnel).
What it looks like from the flap side (this side has to be fitted inside the house). The flap is clear but it doesn't look it in this pic.
Shows the plastic outer cover - if your door is average thickness the tunnel bit will stick out a bit.
An here's what it would look like from outside the house, with Dougie on his chair nearby, thinking "Just hurry up and get it fitted woman"!
Think the hole I've cut is about 1.5cm too high, but am sure he'll manage...