|Photo Credit: IAAF - measuring in Osaka|
One thing the article makes clear--accurately measuring a 26.2 course is a long process, as much an art as a science. The complexity of the London course doesn't make things any easier. Apparently the special course designed for the Olympics has more than 90 turns! That may make for some good spectating but it doesn't make for easy measurement.
If you want to read more about measuring Olympic marathon courses, check out this 2008 article from the Times on the 2008 Beijing course.