To close out this thread… fitted the new Roughtrax RDL. Had a few issues with it not working at first.
I could hear the motor winding on/off with the switch but dash light was just flashing and the RDL actuator was not moving to the locked position inside the diff.
With the RDL switch still in the on position, I started to remove the locker unit. As I loosened the bolts, the clock spring fired and the RDL locked!
I re-tightened the bolts but then it wouldn’t unlock until I slackened the bolts again.
I took the unit off to investigate. The end of the actuator shaft has a small bevelled edge and after passing through the castellated piece (which is fixed to the shaft with the ‘special’ bolt), it seems to slide into a sleeve within the diff.
I wondered if the end of the shaft might be just catching within the sleeve, so I gently filed the bevelled edge to round it off a bit.
I then applied some moly grease to the actuator shaft before refitting the unit.
It now works perfectly. There is no doubt when the actuator fires into position as the clock spring can be heard twanging loudly as the castellated parts of the locker line up, allowing the lock to engage.
To clear up the discussion about diffs and axles, I can confirm:
1. Jack one rear wheel with handbrake off, transmission in neutral and all lockers off - jacked wheel spins freely.
2. Switch on centre diff lock - jacked rear wheel no longer moves.
3. Jack a front wheel in addition to lifted rear wheel and leave centre diff lock on - turn the rear wheel and it moves but also turns the lifted front wheel.
4. Turn on the rear diff lock (which only operates when transmission is in Low ratio) - rear wheel now cannot be moved.
I forgot to try to spin the lifted front wheel at this point, but imagine it would not spin.
I will make a point of engaging the lockers once a month to keep them operating freely.
Having also applied Lanoguard under-body protection to the vehicle yesterday, I painted on a thick layer of Lanoguard grease to the body of the RDL unit, to try to keep the corrosion at bay!