Thanks for all the advice and help from Adobe. Am feeling 'pretty sore' about all this; I asked the reseller loads of questions before purchasing & they insisted they were genuine Adobe resellers (well, they would, wouldn't they!)
Adobe have offered me a significant reduction on a CC subscription but, to a pensioner who was only using CS6 for any actions which could not be achieved in Elements 15, the price is out of my reach.
The good news is, after Adobe phoned & spent over 1 hour trying (unsuccessfully) to get Elements 15 to accept the serial number, I (once more) restored my laptop back to 'factory settings' and made Elements 15 installation the first thing I did. It installed straight away, with no problems. The Adobe technician was 100% correct in his diagnosis "Something in Windows 10 is blocking the installation".
Wish I could go back to XP - had no troubles at all in years of using that platform.
Once again, thank you Adobe for getting Elements 15 up and working for me.