Thanks to all who responded to my request for support.
Lacking the necessary technical expertise to competently delve into the inner workings of my computer , I sought help from the local computer repair shop. I'm pleased to say that a 'permanent' version of LR6 has now re-instated on my computer.
For the record the following is a summary of the workshop repair actions:
- Manually removed Lightroom 5 & 6
- Ran Adobe CC Cleanup Tool
- Installed LR6 and activated with product key
- Updated Lightroom and Adobe Application Manager