This debate is pointless. The admissions criteria are clear and are found at:
http://www.graveney.org/dvportal.aspx?targetportal=17
In summary:
63 Category 1 "general ability" places at Graveney are subject to the Wandsworth test. To get one of these our child can be living anywhere but needs to get a very high score. In the extremely unlikely scenario that any of these places were not filled they are then added to Category 2.
187 Category 2 places are subject to adoption/fostering issues, siblings at the school, medical issues and then location, in that order. In other words a huge block of places, many of which are taken up by siblings. But it also leaves some places for people living near the school, hence the fact there is a catchment area - it is just very small.
As for living on Welham Road, it depends which end you are. The site clearly specifies: "The distance is measured from the applicant’s home to the gates of Lower School, Welham Road, using the shortest route along a recognised public highway." Welham Road is blocked halfway, so presumably houses on the Western side are in the catchment area plus some houses on Chillerton, Crowborough, etc and those on the East side of Furzedown (inc East end of Welham Road) are not in?