Thanks for all the replies. I am struggling to upload photos for some reason.
As commented the drive unit is commonly used on vintage diesels so is not hard to source, don't know why previous owner had difficulty.
What is I think is unusual is that the generator fastens straight on to it using the thread that an outer cable would have used. This means that what you might call the inner cable is about half an inch long and this is what I am missing.
Currently all my engine gauges are Thornycroft branded hence my preference to keep to original. Once you replace one gauge you end up replacing them all and then your messing with 40 year old wiring so you end up replacing that and then all of a sudden your in yet another n thousand pound hole. You know the story, and then the missus sees whats been spent on the boat and wants a new kitchen/bathroom.
Any way I'm retired and don't mind a bit of a challenge.