OK so I took the SD card out again, formatted it with SD Format utility again and burnt the image again. Then rebooted the PI. M2P desktop came up on my TV but it did not connect to my router (Ethernet cable plugged in), so no GUI
Something fundamentally wrong here but I do not have a clue what.
During the boot sequence it does come up with a message in red saying that all dhcpcd services could not be started – perhaps that is relevant?
Well I have given up on this for now. Sometimes it connects briefly on restart then disconnects, but mostly it just does not bother. This is with a new install of m2p on the SD card, so there cannot be a problem with that.
I have gone back to my old Odroid m2p and that connects and works with no problem at all. So there is no problem with my network.
So the only thing I can think is that plugging in a USB optical drive into my Pi3 has caused some hardware damage. That seems most unlikely but I cannot see any other possibility. Can anyone?