Nothing, I can open Winamp/Media Player/VLC/Media Player Classic or any other media player that I choose and it doesn't touch the NAS.
This
must be something specific to iTunes.
I think I'll see if I can find an Apple support forum and ask if anybody else has come across it.
I think I've only noticed it because I allow the NAS to drop into standby mode, rather than leaving it powered on all the time.
Thanks for the help