Thanks for your help Rotund
I considered doing as you suggested, but decided a complete re-install would be less painful!
I noticed after installing N6 trial on the same machine as 5.5.5 that the system was less stable, so have uninstalled for the moment. I seem to now be back where I was before, ie:- able to open existing projects and work!
I’ll probably move on to N6 when they release the next update with more bug fixes.
Cheers