Leaking Basin Waste

First of all check that the large nut is tight enough. You need a tool to tighten this, hand tight is not good enough. If that does not do it then take off the rubber washer and clean off all the sealant from the washer and the basin. buy a polythene waste washer( a few pence) and put it between the large nut and the rubber washer.

What often happens is the friction of the big nut tightening against the rubber twists and distorts it, especially if the rubber washer is on the narrow side.

You're not necessarily doing anything wrong. Plumbers Mait doesn't work in all circumstances and there is a certain amount of controversy as to it's long term effects when in contact with plastic.

I had exactly the same problem. A good clean, making sure all contact parts are dry and an application of silicone sealant in the appropriate places did the trick first try for me. Hope it does for you also.

Unquote

Do you mean not use plumbers mate at all at the bottom just rely upon the black washer?

Yes, that is what I mean. But, don't forget to use a poly washer as well. If I was doing the job then I would do away with the rubber washer and use only the poly washer. I would then use plumbers mait or silicon sealer between the washer & basin.

Clear silicone sanitary sealant normally does the trick.

What i did was apply some sanitary sealant around the underside of the waste. Fit, and nip up the big nut just finger tight. Once the sealant has gone off, nip up further with a spanner.