sorry - I kept it kinda general because of what and where I first beggered the question...

The general consensus there seemed to be that to not respond verbally to a verbal greeting was extremely rude. period. end of discussion. (despite the fact that the particular occasion they were discussing they didn't actually know whether or not the person receiving the greeting had heard or understood the greeting.)

I agree that as I posed the question it is a big "it depends" -