Hi guys, sorry about the delay, work calls!
New sd card generated, installed and upgraded and so far the problem has not recurred.
The moral is to me: in the event of anything strange happening when the B2 has previously been running normally, generate and install a new sd card, it's really easy, then hopefully all of your problems will disappear!
It would be interesting to discover in more detail what had gone wrong but life is too short for that level of investigation....
Mike