Thank you.
And I think I may have found the culprit for the occasional knock from the rear. After taking those pictures I had a feel through/behind the rear wheels for a broken spring. Nothing to report. But the offside cover/sleeve/gaiter/whatever over the shock absorber moves freely up and down so I suppose in the right circumstances it's bottoming out. General driving I hear nothing but slowly drop down a hole on an unmade road or over one of those car park speed bumps and that's when I hear it.
Any thoughts?