How can I fix my leaking mixer tap?

i have a mixer tap and i think the washer needs changing only i cant find where it is,i have removed the plastic cover and unscrewed the metal case to reveal a metal prong this is attached to a bolt and next to this is a metal washer the shape of a horse shoe is this the washer or do i have to remove all of this also to get to the washer and is the washer metal or rubber.
Please help the dripping is driving me crazy.

Lisa
December 2009