I would go along with Blott and book what you want. If you have to pay it will cost you more. Could you get by without an upgrade if you don't get it free and you don't want to pay? How many people in the car; could you use a back seat. We have managed with 5 of us in a full size but it was very packed to and from the airport (but it is only 2 journeys) Last time with the children being older (2 teenagers + 1 child) we had a 7 seater.