I took the swing arm off today in preparation for installing the new parts when they arrive.
I soon found that the problem was no in wear of the bush.
The pivot bar was moving left/right, as opposed to up/down.
A quick stock check of the components, and it would seem that the locking ring, that goes between the bearing and the bolt, was missing.
It looks like it was not put back after it was last taken off by Dockgate 21.
I made up a temporary washer to confirm that was the issue, and sure enough when tightened up the movement had gone, as had the knock.
Tomorrow I will try and cancel the order for the parts, and go visit my trusty nut & bolt man for a washer.