Good news Guys. Problem Solved. [Apologies for the Long Reply]
When I tried to Reinstall MacOS, an error message came up, stating that I needed an extra 2.4G to Reinstall the MacOS. I moved a number of items from the MacBook HDD onto an External Drive and tried again several times having moved more and more items, each time, of the MacBook HDD to the External Drive. Each time I attempted this, the same error, kept coming up, however, curiously the amount of Space required to Reinstall MacOS, kept going up, e.g. 5.7G and then 7G etc.
In the end , having read the Apple Support document HT204904 [How to reinstall macOS], in detail, it became apparent that I needed to erase the HDD in order to proceed.
Having Backed Up my MacBook, using TimeMachine, I proceeded to Erase the HDD as detailed in the Apple Support Document HT208496 [How to erase an Intel-based Mac]. I successfully erased the HDD, Reinstalled MacOS and using the Last Backup Restored my Data to the MacBook Pro. In the end the process is quite painless, but did take nearly 2 Hours to complete [over a 1Gig internet].
I would like thank you all for the Diagnostic Support provided by you guys and would also like to acknowledge and thank Jamie, Kathleen and Alison from the Apple Support Team for all their help and the Support Articles Provide by them.