I think you probably know the answer Simon

As we have said your holy grail is to work on taking the hands out of the swing and using your body to swing the club with a light grip and loose wrists and lower arms

Playing at my club over the weekend one of the guys had got hold of a BEL swing speed radar thing. It was interesting to say the least. Taking a 5i and swinging 'hard' I was averaging about 90-92mph. Then as soon as I loosened my grip and swung at what felt like 80% (certainly much slower than the previous ones) the SS averaged up around 98-100

Being relaxed, loose and swinging easy IS the key to greater distance. These sorts of tests really help to teach your subconcious that 'hitting' the ball is wrong