Selgas
Check that the rubber “O” ring that forms the seal is still intact and has not been kinked or crushed out of shape preventing the seal being made water tight.
Do not overtighten the plastic locking nut as you can crack the base of it and also cause a leak.
Also check that there is no grit or rubbish stuck anywhere on the large rubber seal washer that slides up and down the pipe itself and is sealed with the locking nut.
If the leak is above the plastic pipe where it fits onto the sink outlet waste then it is likely that the sink waste is not sealed into the sink properly – remove same and apply a bead of silocane sealant to the underside of the sink waste recess in the sink then refit the waste outlet and tighten up sufficiently to squeeze out surplus sealant. 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
