Just going to document this here - it's a confirmation of other people's findings which I've seen spread over various places.
1. UNPLUG THE USB CABLE. I don't know why.
2. Switch on with Com button down.
3. Format the disk
4. Follow instructions to upload the firmware.
Appears in my case the firmware had become corrupted (I wasn't running the beta - although I took the opportunity to upgrade to it). I am not sure if the charging helped or not.
None of the above is new in itself. Just a clear run through what worked for me.
Cheers
Ian