I live in a bungalow which has a two-pipe cast iron waste system from
the bathroom. I want to replace the sink/bath waste pipe with plastic.

Currently, this is a large affair with a vent above the gutter line,
like the w.c. waste. Do I have to replace the large pipe with the vent,
or can I just run the waste pipes from the bath and sink separately
straight into the drain?
--
Steve Loft