I've managed to boot my C65GS at last.
I had two problems.
The format of the SD card seemed to be the issue. I used DISKPART on Windows to create a 512MB partition on a 2GB card.
After doing this the hypervisor reported that 01 partitions were mounted :-)
However, it could not find the various files as I had named them incorrectly.
The setup guide I was using referred to C65GS etc whereas the latest release is expecting MEGA65. The file extensions have also changed from G65 to M65. Once the files were renamed everything booted fine.
Note that in addition to the bitstream file the other files need to be named as follows:
Hope this helps.