The results int he little button at the top of this image being useless.

Its a result of the pins on the hinge mechanism, getting slightly bent.
I managed to solve this in about 10 mins last night.
Basically, i drilled & tapped a hole where the thumb button grabs
Stuck a grub screw in, adjusted it to the desired level, Lock-tite and boom
FIXED.
Will post pics if anyone is interested.