[AndyC]

Some of us have reduced hip socket flexibility. This seems to be a problem for me especially on the left side. If the pivot through is restricted, the body stops,the hands flip, and for me, the ball hooks. For now, I'm turning my left foot more towards the target at address, and my shots have started to straighten out.

Anybody know an exercise to improve hip joint mobility?

You might want to look at the TPI site. http://www.mytpi.com/

They will show you how to test your hip flexibility and will also show you a variety of exercises. All at the right price.....free.