Aha- the BIOS needs to be in the root directory of the USB stick! Thanks for the help 
Hi  I'm trying to upgrade my BIOS from version 1.2.20 to critical update version 1.1.23 but I get the error "Driver Version Fail!".  I understand that BIOS versions should be installed one after the other so that there are no gaps however looking at the "Older Versions" on Dell Support there is nothing other than 1.1.23 following 1.2.20 (i.e. no 1.2.21 or 1.1.22)   I've also tried updating via the BIOS update screen (no luck there!) Bitlocker is NOT enabled and TPM is. Any clever people out there with ideas as to what I should try next? R 
BIOS 1.2.23 Upgrade Error: Driver Version Fail!