Hello again. My en suite is progressing nicely and I've just spent an enjoyable couple of hours fitting the hand basin. All seems pretty good except that after around 10 seconds of running the tap, water starts to leak out from the threads where the large plastic nut holds the waste into the bottom of the sink. It's quite a big leak too.
So, a couple of questions if you'd be so kind:

1) It seems to me like plumber's tape wrapped around the threads would cure this - is this a bodge or is it an essential bit that I missed out due to inexperience?  

2) There was no instruction leaflet in the box, and the waste came disassembled. There is a big rubber washer which is sort of "D" shaped in section (So one face flat, one face curved). Does the curved part of the "D" face against the underside of the sink or against the nut?

Thanks in advance for any help you can offer.
Cheers
Dave

.......and leave it to dry before running water down it,  The basin wastes are notorious for this, if iI had a pound for every one I've had do this i'd be a rich man.

i hate those washers - they never seal, especially on ones with pop up wastes where you can't always get it as tight as you'd like and still keep the pop up mechanism lined up - good old silicone