Breakdown of the wrists at top of backswing

I suffer from cocking my wrists too much on the backswing. The angle between my forearm and the shaft is about 45 degrees whereas ideally i want it at about 90 degrees. This makes my iron play very inconsistent.
Because i overcock my wrists I dont complete my shoulder turn as the club would be way past parallel.
Has anyone got any drills to reduce the amount of wrist cock on the backswing because i have been doing this for a long time and its hard to break the habit.
Thanks

I try to wrist cock early during the backswing and it seems to work.

I think that my problem is that i grip the club too loosely so at the top the last three fingers of my left hand let go of the club slightly making it look like my wrists overcock.
I get confused because i hear that you apply pressure with the last three fingers of your left hand but your grip tension should be low like your trapping a small bird between your hands without crushing it. It seems like i have to grip a bit too hard to stop overswinging but ill give it a go. It may be that im so used to griping loosely.

What you are describing is just what I was wrestleing with recently. What I found my problem was that I rushed the swing(both back and through). I have been slowing down my swing and watching my tempo and it seams to have fixed this problem of grip tension.

Now I just need to sort out all my other ones and I'll be ready to take on Tiger

Christopher