What I gleaned from the sites I perused was that the Northala Fields Park is as I suspected a new development. The park opened in 2008 on land which was formerly playing fields and covers over 40 acres and is part of Northolt Greenford Countryside Park.
Now I don't know how I know this but as I am easily led and have been told it more than twice; I now believe it be gospel, the hills which are the dominant feature of the park, were built from the rubble of the old Wembley Stadium.
Apparently the land was offerred as a dump site for a price by Ealing Council and utilsed by the Wembley smashers, as due to the parks close proximity to the stadium, it was a cheap option! Am I making this up, na?