One of these which is the same recommendation as posts 2 and 3
http://www.amazon.co.uk/Junior-Seat-...1919256&sr=8-2
http://www.preciouslittleone.com/acatalog/GRGRJCSM.html
The back comes off (you remove the seat from the car and bend it back below the base then the hinge will separate). So as they grow you can remove the back. Potentially the last seat you need to buy.
As already said, they are held in place with the standard seat belt. This one also has a strap that runs up to the seat belt to keep shoulder placement in place.