Hi all, thanks for the replies - looks like a lot of reading (...but I like reading ;-) )
I have two capture cards that I have working in Linux already (I'm not a Linux noob, just a home automation noob ;-) ) - and I have webcam motion detection setup using 'motion', so I would know when there is someone at the door - just not able to open it without walking down stairs (and after living in bungalows for the last 30 years, my legs "ain't what they used to be" ;-) )
Thanks for the links to software etc... they look exactly what I am after.
...as to the automated cat-flap - I wrote that in vb.net, when I still used Windows - I used a serial cable out to a controller and then on to the servo, which would be triggered once motion was detected. Quite simple really. I did try to get it to recognise only my cat, but this was impossible. I had it recognising members of my family, but cat faces are just toooooo similar to differentiate.
The motion detection I based on some code I found at codeproject.com (I have searched for it, but I can no longer seem to find what I based it on, sorry!)
Thanks again guys - you've really pointed me in the right direction.