The next runway will be at Stansted, Northolt is too impractical for any environmental consideration, noise, ground pollution, and poorly placed to take the additional pax traffic by road into that area of west London. In my opinion the descision has already been made, being at STN regularly over the past few weeks the amount of ground infrastructure under construction at the moment, in advance of last weeks announcement of further development just adds to the development case. Under the Rucatse study STN was again the key player in the SE's development.
In terms of time a public enquiry and hearings etc will take about 2 yrs so next new runway arriving about 2005/6 is my guess. STN's airspace could relatively easily be enlarged to the north of the airfield nicking some off London Mil et al!