After you're forbidden access and redirected, can you see the user menu on the left? "View account, Notifications, Logout" and all this stuff?
I missed this bit ... yes, after redirection I see all that which is the index page for my site. I logged in there with my admin username and password and it let me in then when I clicked Administration it told me I hadn't created an admin yet!
This explains the error at admin.php page; however if I might suggest to the coders that the one line error message could have also said "The reason for this is that you have not yet created a admin login/account. Please login at the index page and then (do this, that, etc. blah blah instructions).
As a noob I can say that would have saved me loads of time working this out.
Thanks to all