I guess this is all that's needed............
http://www.tlc-direct.co.uk/Products/TS012.html
The only problem I can see is that they would be fitted to the outside of the door (assuming door opens inward). I would imagine it would be only too easy for them to be tampered with.
I've had a quick Google, but I haven't yet come up with anything for the inside of uPVC doors.
You'd think the door manufacturers would have addressed this problem by now. Sorry that I have little experience of alarm systems.
As for the salesmen. They're in a little world of their own aren't they? Keep playing them off against each other.