Are you using the internal USB socket?
No. Hadn't realised I needed to. Give it a try now.
Edit - great, well it was a combination of this advice and a few other things, main one was that the usb stick, although worked with the USB bios creation tool, it didn't show up on the Microserver.
Another thing I did was ensure all the files were on the stick as this creation tool didn't copy all files over - I went to where the flat files were (default C:\SWSetup\SP64420) and copied the lot onto the USB drive, replacing any duplicate files (only one for me). I ended up with the following:
Swapping over to a different USB drive enabled me to see and select the USB drive as primary boot in the bios. Restarted and bios update ran perfectly (I used internal USB socket but seen tutorial where external socket used but I wasn't going to argue - it worked).
Once loaded the network card didn't actually work. I read one piece of advice which was to go into bios and select 'optimised settings' but stupidly I missed F10 prompt and loaded straight into W10 - low and behold the network connected and all is fine!
One of the most bizarre issues I've experienced but pleased that after 6 hours I've managed to sort it.
This issue does appear to impact Windows 10, Windows 8 and Server 2012 r2 that I know of and of course the difficulty in getting latest bios with these Microservers is like hens teeth as most people, when they experience this issue, have no warranty left which is the only way HP supply drivers.
Hope this may help someone some day - trust me if you need bios files as they were a pig to find.