The Digger on the flatded was very tall in fact as I followed it off the A23 and under the underpass on to the A273 I remarked to my wife that it only had a foot or two clearance.
The police didn't attend as no one was injured. I think Amahwrang has a good point regarding taking appropriate care, that road has a canopy of trees over it in places and I feel he should have known that the height of his vehicle was unsuitable for that road.
It could be argued that if you'd earlier observed the load was abnormally tall, and then followed it down a road with a canopy of trees, appropriate care would have been hanging back a bit...?
If it struck a branch which fell down and hit your car I'd imagine the insurance company would say you were driving too close. Put it down as one of those things.