Question Author
I upgraded my W8 machine to W8.1. I bought the Windows 8 disk from a legitimate retailer and it installed and ran without problem. It was activated.
I upgraded to W8.1 and it ran well, was activated as it should have been. My hard disk failed completely so I replaced it. I reinstalled W8 using the original disk and licence key then updated to W8.1. I upgraded to W10 and when I tried to use my W8 key I got the 'key not recognised' or 'key invalid' message and I could not continue the upgrade.
I got in contact with MS and they said the key was invalid because the key I was using was for W8, not W8.1. I explained the whole scenario, twice, to two different people. To cut a long story short it was a long winded, expensive process as I had no choice but to buy another operating system after wasting hours over several days with MS.