Although I am still hopefully that I could find the correct parts for now I have done a home fix.
My C5Martin made tensioner was too strong for the the standard replacement part from Karl so an alternative solution had to be found....
I have used the plastic blocks that are used to bolt together kitchen/bedroom furniture but instead of just the one I have stuck (with a good plastic specific superglue) two together so it will not twist off to one side. So far, so good. So now I have 3 working C5's
, two clean(ish) and one needing a good scrub and pair of stripes.
Cheers, Giles