Some further details. I am able to, by going into the INBOX folder, to select mails, unread or not and display them as well as forward or reply to them.
What is NOT happening is that when Mail is active is doesn't display any folder window nor any folder content at all. Thanx for ANY clues.
Embarassing moment... Problem was I have two screens. I guess when I went to sleep last night I decoupled the screens for some reason (probably why I had to go to bed...) and one screen had Mail Inbox displayed. When I tried to display that on the single screen that was turned on this morning, of course, it wouldn't as it was displaying it on the second.
Apologies... do hope it helps someone at some point