80 Series Centre Difflock issues

Ollie, I probably won't be much help but there could be a problem with the diff lock contol relay which is hidding under one of the kick-panels somewhere, but you may be able to hear it click in and out to narrow down the search a bit. I have attached the test diagram from the manual, hope it helps
Chris

Thanks Chris, That's where I was heading. I will have another look.

Try this Put the car in reverse ,back up at about 5 Kph apply brakes. Centre diff locks should disengage. Works for me. regards johny

In the event of power loss to the locker, it will revert to the on position. Make sure everything from the switch down has power.

We had that problem with a friends 80 years ago on a trip, wouldn't unlock and then eventually wouldn't lock either.
Undid the actuator on the side of the transfer case and water had got inside and the little electric motor had seized. Pulled it apart and managed to free it all enough to get it operating sluggishly.
Replacement was the long term solution.