This is odd.
It appears to be a Firefox issue. IE doesn’t have the problem.
Untick ‘Remember passwords for sites’
But the question remains - Why is it retaining my Magento Admin in a Customer Login box??
I need to check it on another machine to be assured that it’s not going to be dropped in by the backend.
 - that might be a dumb question actually - feel free to have a laugh at my expense…