I have a mixer tap that is not dripping from the outlet but from the "gland" where the swivel arm that controls the flow rate enters the main tap body .. see photo. So, the drip is where the horizontal bar below the red ring enters the valve body. How would I progress to get at the O-Ring (?) that's presumably worn inside. There is small screw fitting like the one on the main valve body under the horizontal bar. ..... thanks.