How to identify the cause of entering Safe Mode?
Turned out to be a hardware fault in this particular circumstance a faulty connection between the flex cable that connects the volume buttons and the motherboard... Reseated the connector and 'safe mode' was gone next time I restarted.

Found this by trial and error - don't know of any way to narrow down the cause with the help of software.

So wish I had checked that connector before I went and wiped everything... Doh!