Muddymike writes
We are considering some sort of camera linked to the PC but don't know
firstly if its legal

It's legal but you have to let people know they are being recorded
(which itself might act as a deterrent)

and secondly if a wireless set up would work over the distance and
through the walls of the house. Any advise on deterring teenagers
welcome.

Have you talked to your local police crime prevention officer? Ours was
happy to come round to the house and give specific advice.
--
Kay