Chris Green pretended :
Has anyone come across a video door entry system which uses an
existing television for the display? My M-I-L wants a door entry
system which allows her to see the visitor before unlocking the door
but any 'extra' display hardware really isn't practical.

Display on mobile phone is no good because it probably won't be to
hand and is too small (so she'd need to find her glasses as well as
the phone). A dedicated display doesn't make much sense either,
there's really not space for it anywhere.

It would make *much* more sense to be enable to interrupt the TV
display, it's always on and the *important* thing is that it's when
M-I-L is in easy chair in front of the TV that it takes her a long
time to get up and get to the front door.

Any ideas anyone?

Not a door entry system, but I have a camera.....

It plugs into the audio and video in of a TV in the back of the house,
the kitchen. It has a tiny control unit, with two buttons. One enables
it sounding a tone, the second sets whether the TV is switched over to
the cameras audio and video - both when it spots any motion within its
view. I sourced it on ebay around seven years ago and the image is
640x480. Its output is via SCART or RCA, only the SCART can switch the
TV to the camera's view. Not much use if their are regular passers by
in the cameras view.