You are right Kancha. I am also travelling via London in the end of April. My understanding is that you don't need transit visa as long as your US visa is valid. A friend of mine came to US last July without UK transit visa and went back the same route in the third week of February again. He said he asked immigration people and they also said UK transit visa is not required if the person has valid US visa. But lets keep updating this thread if someone has other contradicting info.