A Phase III, 12-week, Multicentre, Double-blind, Double-dummy, Randomised, Placebo- and Active Comparator-Controlled, Parallel Group study to investigate the Efficacy and Safety of GW406381 1mg, 5mg, 10mg, 25mg and 50mg administered orally once daily, in adults with Osteoarthritis of the knee (CXA30007).