Just to update you all - we have a happy ending. I'm posting this in case anyone else wants to interface a Garmin GPSMAP196 with a Windows 8.1 64 bit PC via USB.
Some time ago (5 years?) I bought a serial to usb converter to connect my Garmin GPSMAP196 to a USB port rather than a serial connector. It precise part number is U232-P9(2.4). Its serial number is 10035P002326.
Needless to say this is what I COULDN'T get to work when I tried to update the GPS and connect it to Skydemon. After vast amounts of trawling of the 'driver' websites and corporate websites of Taiwanese PC component manufacturers, I have established that this cable was manufactured by the Magic Control Technology Corp whose website is at
Magic Control Technology Corp.. I downloaded the driver from them and it worked first time!
So thanks to everyone who helped me out and I just mention this in case anyone else has the same cable and problem. My conclusion is that I am going to use a combination of Skydemon and a GPSMAP196, and that the GPSMAP196 is fully compatible with Windows 8.1 64 bit.