View of the existing catflap cutout from the outside (our cats really are filthy!).

Note the 'ceiling' of the wall tunnel at the top - this needed partially removing to accomodate the sensor coil of the Pet Porte so as not to require the external porch (pictures of which you can see on other installations here and here).

I chose to fit it without the porch for a number of reasons:

1) Performance - the closer the sensor is to the door (and hence the cat) the more reliable the scanning performance. With the scanner inside the wall tunnel the cats have no option but to go within a couple of inches of it and hence it guarantees a perfect scan every time.
2) Functionality - some features such as battery backup operation can only function if the sensor is adjacent to the door. This is basically because in battery mode the scanner only only operates when the cat presses the door (this is done to save power) and so if using the porch on the outside of the wall this feature will not work.
3) Durability - the less exposed the electronics the longer it ought to last (not just as far as the weather is concerned, but the likes of kids playing football etc!).
4) Aesthetics - the porch is not exactly ugly but I prefered to do without it if possible.