I think it's partly a London issue with the way people cycle in crowds - it seems that hanging back and waiting for danger to clear is a signal for some people to overtake a "slower" cyclist, and that some seem to prefer to squeeze between two large white vans or one van and a wall while I'm happier to wait for traffic to start moving again.
But clearly positioning plays a part in how other cyclists respond.
And "whoa" did work today when getting cut off starting at the lights, and a "sorry" from guy wobbling into my path.
Thanks all!