OK, latest discovery - seems as though you (rharv, ROG and LynB) have succeeded in helping me to fix this for which I am very grateful.
What I've now discovered is that whilst Coyote is still not working with existing tracks (only two that matter so no major problem), all seems to be well and working for new tracks.
I did a test project of two MIDI tracks using Coyote as default synth. They recorded and played back as they should and they also rendered to .wav files OK.
I hope I'm not getting unduly excited too soon, but suddenly, thanks to you, what with the ASIO problem also seemingly solved, I seem to be back in business with PT.
It appears that if I were not so technically ignorant and known that you need to still delete stuff after uninstalling, all these weeks of anquish could have been avoided.
Thank you so much - just hope it really is all OK and I don't have to appeal for your help again.
Godfrey