Hi Steve,
I've only just looked at this again today to see your post.
I seem to recall that I had a small adjustable spanner which I used on this.
However, I found that I did not have an allen key the right size to adjust the screw itself, but I found that having loosened the nut, the screw turned quite easily using a smaller allen key held at an angle.
Obviously I proceeded very cautiously with this and would not have done so had there been any stiffness or difficulty.
Mine is a .22. I'm still very pleased with it. Even though I've seen them advertised for £100 less than I paid, by JSR !
And I got a good deal at the time.
Hope you enjoy yours.