North Milton Keynes was an Office for National Statistics designated urban sub-area for the 2001 national census. This terminology is not used for the 2011 census.
The Central Milton Keynes urban sub-area and the North Milton Keynes urban sub-area together approximated to that part of the former Newport Pagnell Rural District that is west of the River Ouzel. The sub-area consists of the modern part of Bradwell, Rooksley, half of Bradwell Common, Linford Wood, half of Downs Barn, Downhead Park, Willen and Tongwell.
The ONS's basis for defining "North Milton Keynes", and its boundary with the Central Milton Keynes urban sub-area, was unclear since it did not respect natural boundaries. It may have related to phases of building developments.
References
North Milton Keynes Wikipedia(Text) CC BY-SA