Rob Winton provides a map of SE23 at http://maps.google.co.uk/maps/ms?ie=UTF8...0d1ba9d055
I have calculated that the geographic centre of SE23 is approximately the junction between Malham and Beadnell Road (or close to the top of Stanstead Road).

If you exclude Honor Oak and One Tree Hill this shifts the centre of 'Forest Hill' to somewhere on Church Rise between Waldram Park and the church (which will please Rob).

Only if you exclude everything east of Brockley Rise can you shift the geographic centre of Forest Hill across the railway line to where the town centre is located.

If you exclude geographic considerations the centre of Forest Hill to me in simply the station. This puts Berkley Homes extremely close to the centre of Forest Hill.