Ok this is fixed. As best as I can tell changing the system setting in BIOS from Windows to Linux bricked it. BIOS didn’t load so there was no display and ‘blind keying’ default setting did nothing – no surprise if BIOS wasn’t loading.
The solution was to unplug the main battery and leave it for a while – 30 minutes for me. After that the F6 booted up again. My best guess is that it cleared BIOS. I couldn’t find a BIOS jumper on the board.