Leaking mixer tap?

I have a five year old mixer tap which is leaking from the top of mixer...I have the one with one spout...do i kneed a new cartridge? or replacement of "O" ring...Also I tried to get tap apart I took decorative cap off but couldn't see the strew to undo...
thank you
my sons friend said it kneeds to be replaced, surly this is the easy way out...he is a plumber!