Looks like I may have resolved the issue.
It seems that there may be a problem with the WIO Terminal fimware on early shipment devices that prevents the uf2 bootloader from working.
But the solution is easy - just load up a (suitable) project that supplies the Arduino .ino and supporting files using the Arduino IDE and it should update whatever is causing the problem with the uf2 Bootloader.
After doing that, I retested the uf2 Bootloader and it all worked ok.