Samba Troubles With My Mac - Password Never Accepted
I finally found the answer on the DD-WRT forums and it's annoyingly simple but I would never have guessed it in a million years.

The username has to be entered in UPPER CASE.  So while smb://share@192.168.1.1/public didn't work, smb://SHARE@192.168.1.1/public did!

Just wish I hadn't spent a day trying to get find that out!