Water overflowing from Toilet Cistern into overflow pipe

[KeithLancashire]

The water level is rising too high when the cistern refills after flushing so the excess overflows into the outside pipe.
Under the old system I could adjust the ballcock arm which I think was copper but the modern cistern has plastic components
There is a white plastic arm then a red joint below it and the blue ballcock screws into the red joint.
How do I adjust the ballcock so that the water stops coming in before it reaches the height of the overflow pipe. :?:

[bobbych]

See where the arm pivots at the valve, just above the pivot you should see a plastic screw which when turned changes when the plunger is closed so closing the valve
I can prob send you a pic if needed

[KeithLancashire]

Thanks. :)
Please send Pic