Hi all.
Juts in case anyone has the same issue, I was updating from a 3.2? version to 3.40 and it fell over during update (I copied up zip file using upload system files).
I had to use method of recovering of copying files manually to sd card and then upgrade firmware using M997 0 and then M997 1.
Error I got initially was
Error: M997: Firmware binary "0:/firmware/Duet2CombinedFirmware.bin" not found
The issue was there was no directory firmware on the sd card.
I created directory manually and copied system files and then was able to complete the tow M997 commands.
Hope that helps anyone who gets stuck.
Not sure if I did anything wrong, but new directory may well catch out people as it did me!