Back when Cardrew used to have thing on the site that TALKED to you, I heard the name being pronouned NEYE-ven.
The Ginster/Cardrew connection is no secret though. When the Advertisement forum was still open on MTS2, Cardrew pedalled its wares under the Ginster screenname.
Edit: Oh my god guys, you weren't even asking how to pronounce Niven! :lol: Sorr 'bout that. But if you ever wanted to know, there you go.