I just found an article on IMDB that Juliet Aubrey wants to return as her role as Helen Cutter.

http://www.imdb.com/name/nm0041411/news#ni1063549

 I wonder how they'll pull that off, since we know that Hellen died at the end of season 3. Juliet Aubrey apparently enjoyed her role so much, she wants to play Hellen again in future seasons of Primeval.