Sparko,
It sounds like your weight is not transferring correctly onto your left foot (assuming your a right hander) on the throughswing. If your weight was transferring correctly and your left foot was holding the majority of your body weight you would not be able to move it backwards/step backwards. Going on this assumption it would seem that your weight is staying on your right side as you hit through the ball.
There is a great drill for helping transfer your weight on the throughswing by one of our online instructors Greg Willis
read it here.
A drill such as this one is an excellent way of getting an idea of the what you should feel in your swing. Practice this drill for 10 mins a day for the next week and report back here of your progress.