Firstly undo the locking wheel at the lever end and turn the cable housing in until there's just a couple of threads showing.
Then, there's a rubber bung on the front sprocket cover, just pop that out.
Ideally you need a deep angled 12mm ring spanner but you can get away with just using a socket to slightly loosen the nut that you'll see.
When that's undone use a screwdriver to turn in the centre screw until you feel it stop, then back it off a quarter turn.
You will then need to tighten back up the outer nut. Now you'll understand why the ring spanner is better for the job, so you can hold the screw in place at the same time, but apparently if you can get the right balance you can still get this done with just a socket without too much movement of the centre screw.
After that's tightened up, pop the bung back in, then adjust again at your lever end 'til there's about 5mm of play.
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