Thanks for the response.
Not sure I can give you the info you have asked for though: the problem occurs when using the restore wizard within SQL Backup.
1. Select "Browse for Backup Files to restore" in the dropdown
2. Select the server to restore to in the dropdown
3. Click the Add Files button and choose the backup file to restore
4. Click Next to take me to Page 2 of the wizard
5. Use the Overwrite existing database option and select the target database
Immediately upon doing this last step, I get the error message as described before appearing below the database selection dropdown, with a red "no entry" symbol next to it.