I used to shout, or rather bellow, "BEHIND!", and I'm not quiet.

This sometimes worked, but on a lot of occasions I got a "I didn't hear you", or a "We've got every right to walk/amble two/three/four abreast and block the path" attitude.

On one occasion two of us running together on a logging path, (wide enough for a lorry), on Cannock Chase encountered two women with three dogs each stopped on opposite sides of the path. The dogs were on flexy leads and completely blocking the path. Despite much bellowing, the women made no attempt to clear the path so we ended up hurdling the dogs. We received abuse for this too.

For what it's worth the same thing happens in cycling, with people walking/ambling down single track roads two, three or even four abreast. I have a piercing bell, and I'm not shy about using it. If they don't take action after four pings then I pick up speed and pass them, which on occasions has made them jump pretty wildly.