When riding with others - a mini-pelaton - I call (like the others do) "CAR BACK!" when there is a car coming up behind.

So on a cycle-path, having tried various things including "onyerleft/right" I find "BIKE BEHIND" to get the best response.

Alot of walkers don't have bikes in their heads as they saunter along and "onyerleft/right type calls just seemed to have them looking around wondering why someone was calling out.

"BIKE BEHIND!" has them (almost always step to the side of the path and then look around...