Thanks for your help Yendor, the problem came back on the weekend so I used your fault finding factory diff lock problems thread to work through what was happening. I moved the vacuum lines to direct vacuum and managed to get the diff lock to lock and unlock this way. There was no vacuum at either the lock or unlock from the solenoid so traced it back from there. Long story short, found the vacuum line that runs to the black plastic canister (assume it's a vacuum reservoir) has a one way non return valve in it and likes to be installed the right way round.. Who knew there was a valve in the rubber hose, a quick suck test either way found the problem.
I suspect the gears in the diff were starting to engage without the unlock vacuum from the solenoid present and that was cause of my troubles. Ready for the Kimberley now :-)
Thanks guys !