Baz,
The red-eye reduction lamp is also the AF assist illuminator "enables the camera to focus even when the subject is poorly lit", so says the manual. If you have the manual you will find this info on page 72.
I'll quote some more just in case you don't have it.
"To use the illuminator, AF-S must be selected for Custom Setting 2 ....., an AF-Nikkor lens must be attached, and the center focus area must be selected or closest subject priority in effect. If these conditions are met and the subject is poorly lit, the illuminator will light automatically to assist the autofocus operation when the shutter-release button is pressed halfway."
To turn this function off go to Custom Menu, option no. 4.
Hope this helps.
Bob