I'm obliged to you gentlemen, and pleased to say all seems to be Ok now following a lengthy "System Repair" which actually finally reported that it couldn't repair one fault, but this seemed unrelated to the fault that was effecting my problem and doesn't seem to be causing any other problems. And no! I can't remember what that fault was.
Fortunately I was able to follow your suggestions on my "other" PC.
One thing I didn't mention; was that the fault was first noticed when I was trying to send emails in Thunderbird and ran into reported failure errors. I was able to receive mail but not send them. This followed my cat jumping on to my keyboard and pressing numerous keys. From that point I went for a two day old "Restore" all seemed Ok until I tried to log back in (see OP) although it didn't at the time cure the send failures, but the System Repair did.
Thanks again.
Experience; is something you gain, just after you needed it most.
When faced with two choices, simply toss a coin. It works not because it settles the question for you. But because in that brief moment while the coin is in the air. You suddenly know what you are hoping for.
