Thank you but it just finally worked on another reboot of the pc:
I got that from the last post in this thread here
communities.vm.com/thread/240391
paste isn't working for me at the moment so I can't paste it here
I had left the box checked and rebooted my pc (quite a few times!) before. It didn't work then. I think it needs two pc reboots in a row.
So here's hoping that the migration is now successful! 12% completed so far. Plenty that could yet go wrong!