I guess my case will be a little different from yours. I booked my car through Toyota Walton Motors last Saturday but they never gave me the customer copy (containing the booking order number etc) giving lame excuses and saying that it'll be delivered to my address on Monday. Didn't happen and I went back to them on Wednesday. There I came to know that they had not booked my car in the first place. They had exhausted their allocation of June and all they did was just to get the draft from me. The booking for cars to be delivered from July onwards was closed owing to the increase in the prices. My Dad was furious as to how they can take the draft if the booking is closed. On asking the reps, they said they had no idea as to when the booking will be formally open again. My Dad asked them to return the draft and as they had not booked my car, the draft was lying with the Finance Department. They returned it back to me
In your case if you have received the customer copy of the booking, I'm not sure as to what is the procedure of the booking to be cancelled.