Inside the cistern is the syphon unit
Between the cistern and the WC Pan is a Doughnut washer & plate
Both of these can be bought from a plumber's merchant or a DIY store.
If you have water continually dribbling into the pan it is unlikely to be due to the syphon or the doughnut washer.
You may do best to pay attention to the adjustment of the inlet valve or the integrity of its sealing washer - also available at the same outlets.
Good luck...
Peccavi
June 2010