How old is your child?

If under 2, you do not need a seat for him/her. You can instead book a bassinet position and use the on-board Britax seat (if too large for a bassinet). You will still have to use a seatbelt extension with the child on your lap when the seatbelt sign is on.

If over 2 you will need to buy a seat. In those circumstances we have never used a car seat, just having the child on the seat, so sorry can't help.