This links to an outline of Surrey's area: http://www.epsomandewellhistoryexplorer.org.uk/images/SurreyChanges.gif

I am thinking possibly that Epsom town centre is where the county becomes east Surrey? A Roman road bisected Surrey, the course of which is the modern A24-A29 route http://en.wikipedia.org/wiki/File:Stane_Street.JPG
But what do people who know Surrey feel defines "east Surrey" from the rest of the county?
These are good answers chaps