This is getting random now. If I go by what's written on their website (- http://www.britainusa.com/visas/other_show.asp?SarticleType=25&Other_ID=313#exempt), I do not need a transit visa. If I listen to people who have actually gone through the route, some say you need it while some say you do not. Do those immigration officers act randomly? Btw, how long are you going get stranded there if you do not have a transit visa? Are those officers willing to listen to you??