Responding to Mark Thorley and thanks to him for his guidance and the picture of the assembly for the horn. I should point out I used to have a working horn and it has recently failed and the steering wheel is I believe original ( leather bound).
We have no good Jaguar garage near for these old cars so I use a local man who likes working on Classic cars he has found the rod but thinks a bit is missing from further down the column. I have an XJS Service Manual but so far have not located the diagram that Mark kindly shows – if he sees this and can give me a source for the diagram ( perhaps a page number) I will resume my investigation for it possibly in my inch thick manual!