UPDATE
I pulled the motor out today and found the same problem, a rivet on the clutch had gone and a sping had popped. The fork looks new.
No HD Defender clutches available locally at the moment. One's due in tomorrow. I'm getting a reinforced fork with it.
While inspecting the engine I noticed a cracked flange (missing one bolt) on the fan pulley, and the bearing felt less than optimal. We used Minnithemoochas homemade flange puller to get the flange off and tomorrow I'll take the cover down to an engineering place and have them press the bearing out and fit a Ford one, and then a Ford flange (same, but machined instead of cast).
I was going to give the timing belt another 15-20K, but since the cover is off I thought I may as well do that. Then when I saw how slack the belt was, I became compelled to replace it.
Also replaced the bearing on the sprung tensioner. I know it's supposed to be a throw away item, but if you unbolt the pulley and drill a couple of little holes in the circlip, then you can remove it with circlip pliers and replace the
6202 bearing. Thanks again to Minnithemoocha for his know-how on that one.