I can't find anything in the PS3 menu, nor in the game menu, that gives me access to the harddrive and/or to deletion of saves.

In the XMB menu, above where you start the game, there's a folder for saved data.

And you press triangle on the file to bring up an options menu where you will see a "delete" option.

Be sure you go into the savegame data and not the gamedata folder as you will be deleting the games instead of the saves.

On the ps3 XMB menu its called "Save Data Utility" i believe. You just go in there and pick the save you want hit triangle and then hit delete