There is no right answer. Pedestrians are totally unpredictable and whatever you do is wrong. If you shout, you are being rude. If you use a bell you are being rude. If you warn people of your presence, they jump across the road like headless chickes or cower by the side till the "danger" has passed. If you approach a group and one person sees or hears you, they wont tell the others. If you approach a couple, they do the Split-S manouvre so you have to ride between them. If a dog hears you bell, he wont tell his owner to reel in the lead.