Additional - it seems linked to locking the screen... told it not to power off after not being used for 1 minute... but then that option would have been ticked yesturday so why now... hmm. So far that seems to have done the trick, though it remains to be seen how this will affect the battery life... it's an
OLED screen so doesn't draw a massive amount of power, and I know I used it yesturday...