[neil]

Basic motion is ok... it's when i get into acquired motion and start introducing the pivot then the problems start. I can chip a 1000 balls and not shank any of them

Try this.
Address the ball,take the club back to the right forearm parallel to the ground position(aquired motion).Do NOT pivot to get to this point.
Right forearm is parallel to the ground with whatever wristcock you normally have at this stage.
Now,keeping your head very still add enough shoulder turn and pivot to get you to the top.It may feel to you like a very "small" move.
Now from this position practice the downstoke waggle,bump the hips parallel to the plane line ,notice how your hands go DOWN ON PLANE.
I too had/have problems with the shorter irons-OVERACCELERATION is my problem,these clubs accelerate very quickly when swung on plane FEEL THE LAG PRESSURE AND SUSTAIN THAT PRESSURE-YOU CANNOT DO IT IF YOU ARE AT ALL QUICK.
Hope this helps as one step along the road.