If it's like my 650 you slacken the wee nut with a size 10 spanner then turn the middle part in with a screw driver until you feel it just touch then back it out about 1/4 turn, then holding the middle screw nice and still you retighten the nut again.
You may need to adjust the slack in your cable too, slacken the cable from the top first then take the slack out from the lower adjuster but leave enough at the top to play with, then back up to the lever end again and adjust that to your own wishes.
