Thanks for all the suggestions. I've tried the legacy updater and it can't connect. It can ping the device but when I start the update it doesn't complete.
Device capabilities in MFDeploy says 'Not Supported'.
There is no firmware on the device as it was erased before the failed update. It's running tinybooter 188.8.131.52.
It is never recognised in serial debug mode. I can ping it in usb debug mode and use fezconfig (the latest) to start the update process (which fails).
I really don't know what to try next.