Hey Lloyd – tough one this. Needs a database tweak, do you have access to phpmyadmin or similar?
If so open up the _options table and search for options_login_logo in the option_name column.
Edit that row by removing the value contained in its option_value field.
When that has been completed go back to your site and refresh your options page you should now have the ability to upload a new logo.
Editing the db would have meant having to get someone else to do it, so I opted to edit the functions file instead as it’d be quicker – and it worked great!
Thanks a lot – I’ll think twice before deleting it directly from the media library again!