Having just (about) completed installation of a new wash-hand basin none of my worries about plumbing the hot and cold water supplies were realised. However, I now find that there is a leak from the basin waste outlet. This is a metal tube with the "plug hole bit" at one end and a thread at the other. It came complete with a plastic nut. I put a bit of sealant round the plug hole bit but just tightened the big plastic nut as much as I dared. It is around this nut that the leak is occurring. What is the approved method of sealing this sort of thing? Sealant, ptfe tape and a few plastic washers are lying around ready to used. Are any of these appropriate?
TIA
Frank [ the slightly less tentative plumber]