In case anybody stumbles across this thread in the future, with the same problem, I will post the solution. The HTPC w/ my custom SCART lead was not sending any voltage down pin 8 to the TV, by design. I discovered the Avant remembers the last signal it received on this pin and uses the same value in the absence of a signal in future! When I connected

I have an old Avant (28", VHS) which is connected to an HTPC via a SCART cable (S-Video + Stereo sound). This is connected to the 'AV' socket on the back of the Avant and springs to life when I press 'DVD' on my BEO4. From the day I built the HTPC and SCART cable it has detected the video source as widescreen and filled the display