poor ride quality rocking feel

I've just got back from a long A road drive. The suspension feels like it slightly rocking from front to back when I was going over a series small bumps in the road. Almost made me car sick. car's at 20k and I've not whacked it over any speed bumps. i know the new diesel is heavier than the petrol but it feels hugely different.

Any ideas? it was like the damping was out. Put in in for a warranty check?

Stu