I tried to update my satnav's firmware again, and this time it didn't stay stuck on 0% but went up. (Not monotically(?), but generally upwards.) And at the end, it reached 100%, asked me to disconnect the device to restart it, switch it off, reconnect it... and when I pressed "Done" after it had re-established communications, it said, "Your software is currently up-to-date."
So I hope that's fixed it.