@phaedrux well I flashed it with Bossa and now it's dead, won't respond to USB-serial or network.
Bossa still recognises it via TTYUSB so something is working but I'm stumped now.
Edit: Re-flashed it with the "boot from flash" option checked in Bossa and now it appears running.
So, probably, thanks! 
@dc42 can you explain to me, as if to an idiot who hasn't read & internalised the detailed workings of the controller, the upgrade process? 
You mention Duet firmware + DWC, as far as I'm aware there's only one board / controller...
I'm guessing the Duet firmware runs on the micro and is perhaps 'RepRapFirmware.bin' that I should somehow be putting on the board via one of the various processes described in the update page, but which one?
https://duet3d.dozuki.com/Wiki/Installing_and_Updating_Firmware
or
https://duet3d.dozuki.com/Wiki/Installing_and_Updating_Firmware_(Legacy)
and which part(s) of those?
Is the DWC just a web application served from SD card and updated via the web interface?
I've been gifted an old Ormerod with a Duet 0.6 board with web controls.
Firmware Name: RepRapFirmware for Duet
Firmware Electronics: Duet 0.6
Firmware Version: 1.20.1RC2 (2018-01-01)
Web Interface Version: 1.20
I want to upgrade the firmware if I can, I've found a thread here about the latest compatible firmware release but the upgrade instructions page has got me confused as to which parts I should be following, which files I should be downloading etc.
Any help appreciated!