There is a way to do it and I know that TW tech support do know it although thankfully I haven't had to.
I think the mechanism is buried somewhere in
this forum but could't tell you where. (Sorry!)
The other reason I know it's possible is because we found our TVD had been sent some sort of hard reset hit at one point in the middle of the night - which forced it back to factory default settings i.e. SCART.