Mixer taps, by their very nature, tend to more complex, because there are three potentially different sources of leak and therefore three potential areas for repair.
Due to the fact that mixers supply both hot and cold water, each is subject to leaking, as is the spout of the mixer.
Remove the spout from the body of the tap by releasing the snap-ring, which is normally hidden under the cover. This can be unscrewed to release it or in some cases by checking at the back, where a small grub screw might be located, which must be undone to allow the spout to be lifted off.
If you look under the handle or knob you might find the flats that accept a spanner. If so, undo this nut and the mechanism can then be removed.
The washer is normally found between brass flanges and can be easily accessed and removed. On some models there might be two O-rings, one large and the other smaller. Again, these are easily replaced and are readily obtainable from a plumber’s merchant.
The washers and seating on the hot and cold sections of the mixer tap can be dealt with using the same method as you would for repairing normal taps and details of how to do this can be found by making reference to the section on Tap Dripping.
When the repairs have been completed, the mixer spout can be replaced in reverse order and the problem should be solved.