On birdcage walk south of St James's park. In free roam mode from where you start do a 180 turn and drive until the end of that road, take a left then a right then a left and a right. you should be faceing wellintons monument on Hyde park corner take the first left and follow it until you see Buckingham palace on your right. cut across the black posts and take a left you should now be on birdcage walk, the tank is on your right hidden behind a fence if you dont se it the first time turn around and go back the same way.