How likely will it be to find a space on a Saturday morning?
Could you let us know a little more about your itinerary surrounding this?
Are you driving to Bath, and want to take the train?
Do you simply want to park your car?
Before 10am you should be fine. Are you actually using the train? The parking across the road under Southgate that is set aside for rail users requires you to validate your ticket at the train station.