its from b + q £189 suite, a plastic inlet thread comes out of the
cistern to attach the water supply.

the pipe coming up to the cistern has a white plastic connector with a
black rubber washer and gold metal nut. when i put everything in place
and tighten the nut (taking care not to cross thread), its still
diffficult to tighten it up, and after a few turns with a spanner, it
doesnt look straight.

with the water on theres a very slow leak, a little drop of water
seeping from under the nut occasionaly.

now is the problem using a metal nut on a plastic thread? the washer
looks in good condition. any help appreciated.