The Extron is in mint condition and sold as fully tested & working by a highly rated eBay seller. I'm using a good VGA lead and the BNC cable is from retrogamingcables.co.uk who have a reputation for good quality cables. It's made very well. I took it apart to check that sync is wired to pin 20 on the scart plug (~which is sould be as it's an output) and it is:
I've got it set up like this:
But I actually have the yellow sync wire plugged into the 'S' sync output (apparently this is the right one). As you can see, the Extron is registering the Sync output from the Sync Strike:
Does anyone know what could be preventing me from getting a picture? Is there a good way of isolating the problem?