Thanks RickB - thats a good thought.
I have no manual for this machine so I don't know exactly how the clutch mechanism functions mechanically, but I can see that a worn plate could potentially affect the point at which the pto clutch would start to engage, and that adjustment of the mechanism could possibly re-establish the correct operating point. I have already tried adjusting the pedal linkage in case it was simply a case of lost motion stopping it reaching the disengage threshold, but it feels as though all of the internal movement available within the clutch mechanism is being used, which also lends support to your suggestion.
Can you elaborate a bit about the internal adjustement you refer to? There is an oval cut-out about 3"x2" in the bell housing which is accessible already without removing anything (I have a Lewis front loader fitted, which attaches to the forward pair of fixings and does not obstruct this opening). I have previously studied that to see if there was any scope to insert something between the plates via this opening to pry them apart, on the asssumption that they were sticking. Although I could see movement of the clutch plate when the clutch pedal was depressed, I could not see any way of getting access to anything internal to the clutch. What should I look for regarding the adjustment you suggest, and what exactly did you have in mind?
After posting my original query, I found some earlier threads on this problem from several years ago which confirmed that the pto clutch is prone to sticking if not regularly used. Suggestions were made as to how it might be freed - mostly by various shock-loading methods, which I had already considered but reluctant to try for fear of breaking a gear.
I am not keen to split the tractor as it is working very well in other respects, and at the moment I need both the back hoe and the loader daily. Might be sensible to look at buying another tractor just for haymaking etc, but its annoying not to have it working properly.