Having just downloaded and installed the latest versions of the game and its DLC (.22), when I launch it I get the Shipwrecked game screen. I would prefer to start with the original game first, so how do I do it? Or is it just the game screen that is confusing me and will starting a new game start at the beginning of the beginning, so to speak?

When you start a new game there are two X boxes, one for ROG and one for Shipwrecked.

Just unclick both of those to get the base game. IMO it is a good idea to start without the DLC, as ROG at least ups the difficulty and learning curve quite a lot.