Ian,

I suffer with this shot sometimes but, nowhere as much as I used to. In my case, it's a block to the left, being a leftie.

The standard response, according to the pros, is that your clubhead is arriving at impact on a swing path that is too in-to-out. I accepted the "experts" opinion and spent hours trying to make my swingpath squarer. The block, that all the books told me was being caused by an extreme in-to-out swing path, was nothing of the sort.

It may be a simple case that you have lost the synchronisation in your swing. The cause of my block was, purely, that my head was not staying behind the ball at impact. As a result, my left shoulder was not moving down and under, in the downswing. Therefore it was almost impossible to get the clubface square at impact.

Check this out. If you have a practise partner, have them stand in front of you. Get them to hold a club, by the head end, so that it is horizontal and stretch their arm out. The grip of the club should be placed against the left side of your head. Take your swing and you will soon know whether you are moving your head forward in the downswing.