Okay so I've had some free time to take a look a the database. I really wanted to undo my decision on blocking certain Npc's and dungeons.
I've now managed to retrospectively add both dungeons and Npcs without regenerating the world.
If anyone wants to know how to unblock or block certain Npcs / Dungeons *after* the world is created, open up the games database and check out the table WorldInfo, in there you will find two fields (amongst others): DisableNPCs, and DisabledDungeons, these VARCHARS contain the Strings with the blocked Npcs and Dungeon types.
Simply edit these strings, save changes, and then restart the MySQL service.
I have not 100% tested but I did go into my world and went into a newly generated chunk and I did indeed find a Polar Bear! Which previously was blocked.
Update: All tested and working as expected in regards to animals, found some land based dungeons, but nothing subterranean yet. . . I'll keep looking (and updating).
Next will be to retrospectively add caves.
UPDATED: Caves working too!
Many thanks to @Minotorious for the idea! 