I sort of found what I wanted - I've used one of the combined security light/PIR with a built in camera - they usually go for around £100 but found them on sale for £60.
Typically they record to an SD card when the PIR senses movement but what I've done is use the mini-usb connector and not bother with connecting up the power for the actual light & PIR sensor.
The CMOS in the camera is supported to a degree under linux and I've got it working well using ubuntu 8.10 (though oddly bugged in earlier and subsequent release) and have configured the application 'motion' to capture avi & jpgs based on perceived motion. This seems to work well and I've added a mask file so it ignores any movement on the public path, so will only trigger based on movement on my property.
All-in-all its been more of a fun exercise than any 'serious' security, but I've learned a bit
The light/camera isn't noticeably any different from those on loads of houses on the street, and I think 99% of callers to the house wouldn't notice the camera part unless they were looking for it.
Heres a picture of how it looks,
and a capture from the camera of some lovely person delivering junkmail,