So, I deleted all the phpBB files and dropped all the tables in the DB.
I reinstalled phpBB, using a different username and password, and set about trying some customising of a theme.
I can now login in Opera but get the attached error in Firefox. Scratch that, cannot upload an image here. "The submitted form was invalid. Try submitting again.", which I have done several times now.
My caches have been cleared 100s of times today.
If phpBB is browser specific we cannot use it for a public forum
I have used phpBB before as a forum member elsewhere, which is why we decided to trial it. I had no idea it would be so difficult to install and get working