I'd advise doing a full day, not a taster the cost of the day is relatively immaterial as others have pointed out as the wear and tear on the car is more and fuel. Though starting out will likely be less burdensome until up to speed. Blyton (lots of cheap weekend days) or Bedford have lots of run off.
Also I would advise getting the most out of day seeming as you paid for it, the number of people who pack up by 14:00 on most days is crazy. Fill up car before getting there in morning and again at lunch time to ensure enough fuel, once you get into it you'll need jerry cans as well.
edit to add - take lots of tools and fluids with you just in case.