Why not stay in Dartford somewhere. There's bound to be a premier inn /travelodge. Not the nicest town in the world BUT you'll be on the railway line that will take you to ANY of the three starts AND you won't be on an overcrowded train like the London ones. Believe me, I've done it and will do so again!
A number of you have mentioned you think its going to be a hot one. Has anyone got an official forecast yet? I did the 2007 marathon as I'm sure many of you did and, because of the heat, slowed up by around 20 mins! Don't want to repeat that on Sunday if poss.