I am happy to report that this now seems to have been cured, as well as another problem recently encountered with CoyoteWT.
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 is 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.