OK, bit the Bullit!
Managed to get 8.0 to install on another "old" drive, fiddled the Key as per suggestion elsewhere and then installed 8.1 and changed the key to my valid 8.0 key.
Of course my PC is now back at square one and it is possibly a good time to re-install all the apps I really use and get rid of the legacy that accumelated since I built this "Super PC" in 2009/04.
Still knocks the socks off the "High-end" units we get at work! (i7-2600 3.4GHz 4HB HP 8200 Elite's)
Good Luck to all the others still struggling - there does not seem to be an easy way on some systems!
Thanks for your contributions - Learnt a lot - You never learn anything if whatever you are busy with works 1st shot!