If the base of the plug that fits against the side of the porcelain drain hole isn't sealed properly it will leak as you've described. Sometimes the drain kit has a rubber gasket, but more often you need a product called "Plumber's Putty"... it comes in a can (I'm in the U.S., by the way) and is the consistency of children's play dough. Roll out a length (between the palms of your hands, just like a kid) long enough to go around the base of the drain and about the size of a pencil. Place it around the base, insert the base in the drain hole and when you apply force to the nut on the bottom, a lot of the putty will squeeze out. Tt's easy to wipe out the surplus and you should be good to go... Best of luck!