Thanks to everyone who posted suggestions. I appreciate your help.
Having now disassembled the rudder with my AME, the answer seems to be that the retaining nut was too tight.
The rudder on this ship is kept in place by an M8 metal locknut and washer on the lower pivot pin. On disassembly, the nut was hard to turn at first. After loosening by 1/2-turn the rudder moved freely. Upon removing the rudder the bronze bushing and washer on the lower pivot had bright mating faces showing sliding contact.
Not so clear is why the nut had become overtight. The rudder never been taken off. One theory is that the nut worked itself tighter through friction as the rudder moved.
No significant corrosion was seen on the rudder horn, bearing or mounts.
On reassembly after cleaning and lubrication and with a new metal locknut my AME could reproduce the issue by tightening the nut against the bearing. After backing off the nut slightly to give slight clearance the rudder moves freely.
Some pics here if anyone wishes to take a look. https://1drv.ms/u/s!Al9GDrAowZRAi59D...aIqCA?e=jhOKn8