I found a key but when I try and turn it anti clockwise, it seems to twist a few mm then "clicks" and feels like the key has popped out. Manage to turn it a few mm each time before it "clicks" and slips again.

Dont think im actually moving anything as the tuning doesn't change and the neck hasn't got any slacker.. hmm.

I might need a heavier duty allen key. this one looks the right size and seems to initially fit but its really hard to see.