Hi, the new legislation doesn't actually come in until Septemebr 18th. All children under 135cms in height are affeted, up to the age of 12, and must be on a booster cushion or seat. A booster seat is one of those which has a back but uses an ordinary seatbelt; a booster cushion is just the bit you sit on with no back. If you have three children in the car and all are under 135cms, and you can't get three car seats or boosters across the back seat then one child is suposed to go in the front with a booster or car seat. However, if you've got an airbag at the front and you don't want them to go in the front because of it, then you are allowed to let the tallest child sit in the middle using an adult belt only.
must be a bit of a nightmare if you've got more than two children under 135cms.
The AA has a good piece about the new legislation on its website.