I just had this problem with a Belfast sink. In the end, I did the
following:

1. Fill threads of waste with Boss White.
2. Wrap loads of PTFE around threads.
3. Insert waste into hole.
4. Put big continuous bead of silicone sealant around rubber washer.
5. Push rubber washer up against bottom of sink.
6. Put big bead of sealant around metal nut.
7. Tighten nut, hard.
8. Wipe off excess sealant (i.e. loads of it)
9. WAIT FOUR HOURS.
10. Test with water.

This is a technique I've used before. If I stray from it, I always get
leaks. I have no idea what the correct technique is, because the waste
fittings never come with instructions. There is a wealth of information on
how to do push fit, copper soldering or compression joints. There is never
anything about waste fittings. It is hard enough working out which bit of a
waste go which side of the sink hole. They never seem to come with
instructions, or even a diagram.

Christian.