Hi Andy, I'm a beginner myself so I am just guessing. If your left foot is moving that much, it could be that you are not transferring your weight to the left foot after impact and at the follow through. Do you hands go forward after impact, or do they just fold around the body? You can try extending your hands in front of you after impact, and that should help transfer your weight more to your left foot. One drill I've seen is to actually literally throw your clubs to your front, so you learn the feeling of how your hands and weight should go forward.

I've seen some golfers' habit, where after impact, they are actually spreading their legs and you can see that they actually take a step back with their right foot. That's because they are not transferring their weight sufficiently from back to front.