Okay I now have a bit more information after speaking to my grandmother. Her father was John William Noel Grenfell born in Sydenham, Christchurch, New Zealand. He married a Mary Aileen Rookes or Rooks - also of Christchurch, NZ. His father was John Grenfell, and mother was Decima Hall Duttom.
It was either John Grenfell Sr's father or grandfather who originally came to NZ from Cornwall. If anyone has any information, my e-mail is email@example.com. Or perhaps a way to find out some further information online.... ? Any help would be most appreciated - finding links to your family history is hard work indeed.: )