loose and leaking mixer taps on kitchen sink?

hi i have mixer taps in the kitchen that are constantly loose the whole unit can be wobbled and turned and water still comes out the tap when its turned compleatly off ive tightend the fixings underneath as much as humanly possible without it cracking but its still loose and still leaks any advice would be very appreiciated seals maybe?? many thanks kurt

Kurt
March 2009