I have just fitted a new sink in my cloakroom which has one of these
click/clack waste fittings, with the overflow running into the slotted waste
fitting The plug fitting drops through the hole in the sink, with a plastic
washer between the plug and sink, with another plastic washer/seal that fits
under the sink with a large nut to tighten the plug to sink joint. My
problem is I cannot tighten the nut enough to prevent water running down the
thread of the waste fitting below the sink. I'm guessing that the water is
finding a route either between the plug and sink on the top side, or
possibly through the lower washer ? Is there a better way of sealing this ?

Thanks
Tony