Gaz,
I'd go with the thinnest material I could find. As a general rule I use invisible sewing thread for most of my rigging.The thread is available here in both clear and smoke colors. I do once in a while use EZ Line or Aeroclub's rigging thread ,but only for the exterior . I won't use it internally because I tend to pull it too tight,sending rudder bar and stick off into space in a snap.
Highest Regards,
Gregory Jouette