What kind of F number were you shooting at Rich?

The wider the aperture, the quicker the shutter speed, the less blurred it should be.

This may sound daft, but it does work. It's worth trying to perfect your panning technique by taking pics of passing cars on a road near you. It'll get you into all sorts of good habits, panning, aperture & ISO settings etc.

One other question while I remember, what type of focusing did you have on? Was it on continuous?