Thanks for the replies guys.
Yep..definitely an opto isolator to trigger the circuit.
One thing I did notice about these switches was that they not only switch based on the button, but also if there is only one live input. Presumably they sense the signal somehow. Unfortunately this isn't desirable in my case... As this would bypass my Arduino logic.
I might still get one and crack it open... Could still be an option if I can disable the auto switching function.
Re the transistor option... I don't quite understand how this would work. I can see that it provides a switch but don't quite see how I could use this to pass the signal down the cable.
Thanks