Hi, refitting out our shiwer room, new sink and vanity unit, fitting the drain, trap etc today. The part in the photo that attach to the sink itself is leaking from the connection to the sink. Water dripping down the threads onto the trap etc below. Tried everything, still leaks. Added some silicone seal to the rubber washers etc, no difference so ceaned all off thoroughly. Assembled again using different pressures to tighten the big plastic nut, still no difference. Connect to the trap and beyond all good. Never had this issue with a sink fitting before, I've done a few over the years, usually a very basic connection.

