Many thanks for your prompt response and intelligent suggestion. However, I haven't installed any third party configuration software, so I think that can be eliminated as a cause. I am beginning to think this could be a configuration conflict as the problem coincided with the installation of VFlyte Air's Cirrus SR20.
I have now completed yet another re-installation of X-Plane 11 in the hope that I can start with a clean sheet. I will not re-install the SR20, which I didn't particularly like anyway. The stock SF50 Vision Jet is a much better offering.
My Flight Simming stretches back to Sub logic days, despite which my enthusiasm is undimmed despite the fact that I am about to begin my ninth decade.