Also beefed up the dig spring - it wasn't always reverting back to 4WD although it was disengaging dig.
Not sure if it was just that all the sliding parts were new and needed wearing in or not, but it was last night's project to get it sorted
I didn't take pics as it took several attempts to get it right and it was a bit of a patience test
What solved it was a sandwich of 3mm washer, o-ring, washer, o-ring and another washer between the spring and the shift fork.
The washers stop the o-rings stretching around the spring or the fork moulding - I tried without washers first and the spring was forced thru the o-ring stack
The blue and yellow bits in the pic represent the washer/o-ring stack - solved the problem

I didn't want to drill/ream/modify anything irreversibly in case the assembly just needed to bed-in
This way, if the assembly does loosen up the mod can stay in place