DISREGARD Stig's advice on tools to use please. Only use a 6 point socket or a 6 point box wrench. If it's a square head only use the proper open end wrench.
Yep, all you need is a 3/16th or 1/8th inch open end wrench and you're all set to have at that valve adjustment screw.
OR, a wire nut, or one of those fancy jam nut valve adjustment tools, or your teeth, or your right thumb and forefinger.....because once you loosen that jam nut (10mm??) with a metric box wrench----you'll find that there is very little holding that square headed screw in place.
Ordinarily.. I'd be leading the parade of Stig naysayers when it comes to mechanical stuff...but, sadly, I sorta know what I'm talking about this time....as I have been adjusting valve lash for a few decades. You need a box wrench to loosen the jam nut....then pretty much anything (clothes pin ??) to hold the screw. NO special tool needed.
Set the gap, hold the screw, tighten the jam nut. Check gap again after spinning the engine 360.