Back online after some time...
I have it working; it was the USB stick ; which I formatted MS-DS on a Mac; and despite having formatted it MS-DOS, the Volume Partition Map Scheme i.e on the parent volume itself, was GUID.
So the moral is, the USB volume needs to be
1. formatted MS-DOS
2. using a MBR partition map
Once this was corrected, the BIOS updated ok
thanks again for the help