Here is a picture of the place where the hole used to be. You can see that it snapped across the hole and reflecting Nicks comment below I was trying to adapt it as the large hole was allowing the spring to jump off. I have looked for the remaining bit, but it shot off into the garage ....
For reference the spring length (measured with string) is 44.7cm, 1mm thick. 9mm outside diameter of locking ring.
Happy to investigate potential spring suppliers to see if they would be interested in making a small batch if there are no other spares available.