If it is the type that I think which is a mixter tap with a round top then chances are that the water is leaking form the body of the tap
First turn off the water and run the tap to remove any pressure from the tap
On the front there a small allen screw, remove this and be careful that you put the plug into the bath firts so you don't drop it down the plug hole
Pull the top of the tap off and then check that the main plastic body of the tap isn't loose, turn on the water and you should be able to see where it is leaking from, normally around the edge where the plastic meets the metal edge
Water off, run tap to remove pressure, and unscrew the body the re-seat it and tighten it up, replace the top and tighten the allen screw, which should solve the problem