Thanks for the input from all. Bigted is right about the blade carrier Norm but I know what you mean and I have had improvement on other mowers by changing a bar blade. I think I've found the culprit and AVB is on to something. I believe the ACR spring has come adrift. In one of the photos I am holding the camshaft and I hope you can see the spring. It has no tension on it at all. I presume it goes around the back of one of those movable hooks but the spring hook itself is actually bent and has slipped off. Some photos:
