OK!! woohoo!!!!!!! I think I have cracked it
After about 5 hrs of banging my head on the top of my G25 wheel.
Win 10 completed a re-install and fortunately did not remove any of the folders on my HDD, so I was able to reconstruct my desktop.
Firstly thanks to all for their help and advice, especially to Tony and Matt for offering their GTL downloads etc.
I have installed Tony's GTL copy as a virgin install ( because it arrived first) and that worked fine OFFLINE with all the std cars and circuits. But, I just could not get access to the lobby no matter how many times I followed the various tutorials on the forum. I tried new identities and p/words and even used an old email account that I know is still acive
....I got asked for the online key, but never an opportunity to enter the code.
I knew that I had my copy of GTL logged on Steam, so as a last resort I though I would pull in a fresh Steam download and try that. But Steam decided that the 2.7 TB of empty space on my HDD was insufficient for the download and asked if I could clear some old files to make room!!!??
Then I discovered a menu on Steam with " verify integrity of game files " as an option, so I figured that I had nothing to lose by trying that. It produced an alert to 3 files that were corrupted and repaired them. Unfortunately it did not say which files were wrong.
I had kept the old GTL folder that carried EVERYTHING we have used since I joined UKGTL, and that was the GTL that had received the repair. So I fired that massive folder up and it came to life. I was fully expecting to have a 68 1275 mini again, but woohoo!!! I could select any car and drive it.
Then I tried the Multiplayer/Internet option and sailed straight through to the lobby, where I could access the Heartland practice. Strangely, when I tried to insert that into Tony's version before it would not work
So I figured that I must now have a .plr file that was valid ( I may be old and dumb, but sometimes I have lucid moments!
) So, I deleted the .plr file in Tony's clean version and copied the one that I knew worked into that install.
So I ( think) that I now have a clean/virgin GTL install with a valid connection to the lobby. Plus I still have my old install working with all of the setups etc, which I can transfer when required.
Next job is to make a copy of the clean install and stuff Tony's Heavy Metal download into it and then try on-line again. But that will have to wait until I have walked the dog...he has 4 legs crossed and is whimpering!!
Thanks again to everyone.