(Adds quotes) By Martyn Herman LONDON, March 5 (Reuters) - Mark Cavendish's hopes of claiming a place in Britain's Olympic team hang in the balance after he finished sixth in the omnium at the world track championships on Saturday. The 30-year-old was fifth going into the final discipline, the 160-lap points race, and was unable to improve his position as Colombia's Fernando Gaviria Rendon registered an epic victory from charging German Roger Kluge. Earlier, Cavendish finished 11th in the 1km time trial before reviving his chances in the flying lap where he was fourth quickest. More...