The problem with "On your left" is that the pedestrians didn't know you were coming and have only a few seconds to process what's going on. They don't encounter enough cyclists to have an automatic reaction so their reaction will be unpredictable (by the time their mind has processed "left" you're already there). So I ring my bell once or twice and slow down, they may go left, right or split but they understand they need to get out of the way. It seems to work very well in most cases. (And then I say "Thank you!" as I go past).