When Stella reveals she's pregnant, she claims it must be Nathan's as he is the only one allowed not to use protection. However, in series 1, when Stella is having an affair with Dale she states that Nathan can't be allowed to find her pills as he has had a vasectomy.
When Saint first introduces Kim to ex-boyfriend Mark, she mentions that Kim is currently studying for her O-Level exams. However, O-Levels were discontinued in England in the year 1987 (replaced by the GCSEs). In 2:9, Sugar mentions that she hasn't been to the theatre since 1997, meaning this particular series is set in (at a bare minimum) the year 1998.