Rockshox 35 lockout moving freely

[entropy1024]

I have a Rockshox 35 front fork and the lockout dial on the right post tends to move from the 'suspension active' position to 'lockout' all by itself after riding on rough terrain for a while. Pretty much the opposite of why you want.

Is there a way to make the dial stiffer to stop it moving so freely?
Is it simply a case of tightening the retaining screw on the top of that tube?

Many thanks for any help.