I'm pretty sure I had uploaded 6.34.6 to it at some point, never got around to updating it. I hadn't updated the firmware from 3.02, so when it upgraded from 6.07 or whatever it was the old bootloader couldn't load the new OS.
Reset to defaults (probably 4 times) after netinstalling 5.26 and I'm back up and running.
I could have done without this after the week I've had.