Bristan mixer shower runs lukewarm

[Ac71]

Hi..just moved in property and noticed the shower doesnt run as hot as i would like basically a little hotter than lukewarm,
after turning temp dial to max with red button down..its fed from a combi boiler and
the bath tap runs hot as it should, so obvs somthing to do with mixer.?.
anything i can do to remedy it..pic included of mixer ..

 

[Ray1000]

Is it one of those cases where you could remove the control knob, perhaps held on with a grub screw, and move the knob round on a spline so it effectively turns the spindle further towards hot