I've just gone out and bought a "SanDisk SD Card - 2GB"
Used EaseUS to format it as Fat16, 64K, Active Primary partition.
Loaded the files from the Dynavin website onto it, put the card into the "Navi" slot on the Headunit and rebooted it.
Again, nothing happened. It just booted up as normal - Android see's the card OK, on first boot up it said at the top bar "Preparing SD card..." and in the Settings-->Storage it sees the SD card fine.
I am waiting for someone to send me across a format utility that Dynavin sent him, so hopefully that will help - but none of the above has worked for me so far.