This new bug report has not been dealt with, but I have marked it as fixed, as it seems to be a vanishingly rare occurrence (only one other user reported anything similar) and the root cause has been dealt with here.
I include this information here however, for the record.
This bug has caused a situation whereby I will probably have to reinstall Ubuntu in order to get the up-to date kernel running on my hardware.
See comments 38, 42, 51 and 53 for details.
I filed another bug report to describe my particular situation:
https:/ /bugs.launchpad .net/ubuntu/ +source/ xorg/+bug/ 1486673
This new bug report has not been dealt with, but I have marked it as fixed, as it seems to be a vanishingly rare occurrence (only one other user reported anything similar) and the root cause has been dealt with here.
I include this information here however, for the record.
Thanks,
Rich