The last 2 or 3 years you have been able to track runners via the london marathon website under the results section -
http://www.virginlondonmarathon.com/marathon-centre/race-results/race-results/
(sorry link thingy isn't working)
Not sure if it'll be available this year with change of website but if so, just click on 2010 results (should appear when race starts) type in runners number or name and their time will appear as they pass each 5k as well as their half way split.
If you just type in their club with no name/number it will show all runners competing under that club name.