Citigo Clutch Adjustment

I am becoming more convinced that ongoing gear engagement problems with my Citigo are due to inconsistent clutch operation. Sometimes the pedal feels a little 'odd' and this coincides with grating gear sounds. I know that the clutch operation is hydraulic, but I can't see any fluid reservoir, and various online searches have yielded no information about how to adjust the Citigo/Mii/Up clutch.

Does anyone know how to check/adjust the Citigo clutch?