Here I am !!!!!!!
OS 'invited' me to install 'without any changes' on to a drive I use for personal stuff. Dead straightforward after I disconnected the drive that would not boot. Now all I have to do is spend the next three weeks putting things right!!!!!!
BOAC was more than helpful, as usual, and spent a long time on the phone with me. And....we have a crafty plan to avoid such a problem again. Love to know what caused it though. One minute a perfect drive, next it would not boot.
Thanks for your suggestions.