probs with rear suspension knocking over bumpy roads?

57 plate corsa 1400 sxi is it possible to fit 'softer' shocks to the car as ride is really poor. when in warranty car kept going back to rid this knocking and they would sort it but wouldnt say exactly what the prob was.i think the shocks are so hard that things just start to give after a while. any thoughts would be great.cheers