Saying Left is confusing- people think you are instructing them to go left.
As I've said before- when people come up behind me and say "On your left!" 90% of the time all I ever hear is "mumbledy-mumbledy LEFT!". And I have good hearing.
THAT IS WHY pedestrians keep jumping left on you!!

I ring my bell ahead of time- 50% of the time the person ahead glances back and knows what to do and I needn't do anything more except thank them as I pass. If they don't glance back at my approaching bell, I say strongly "bicycle passing you!" or "Bicycle Coming!" with another bell ring, and "Thank you" as I pass them.
If I am approaching a situation where I'm not technically passing someone but there is a group talking and blocking the traffic pathway- I say LOUDLY with my bell ahead of time "Look Out! Coming through!!" that usually gets them to all step out of the lane. I always say things loudly and clearly with an air of confidence, but in a CHEERFUL FRIENDLY tone.