Well, after my frustrated rant, suddenly things seem to have taken a turn and I'm happy to report that my problems seem to have been cured (hope I'm not speaking too soon!).
I was advised by members on this forum that when you uninstall the program to re-install it afresh, the uninstall does not remove all PT files so you need to go through and manually delete what's left before re-installing - I didn't know that. Having now performed that process, all seems to be well.
It appears that if I were not so technically ignorant and had known that earlier, all these weeks of anquish could have been avoided.
My grateful thanks to all who helped me, and to Rob at PG support who just called again to see if he could do any more to help.