When the second salesman was there, my TV was on and audible. Like a vampire, they need to be invited in first so they can inspect the equipment. Because of the notice, he was more concerned about the civil trespass and left.
Traipsing around my back garden with a video camera and peering through my lounge window might be one way of getting evidence, but the notice prevents that.
Inviting them in is the most common way for a prosecution to go ahead.
You can google one of these "anti-establishment" websites at your leisure!