Every year in the UK, around 30 children under 12 are killed in cars and a further 5,000 are injured. Little wonder when you consider recent surveys such as Gocompare.com's, which found that only a fifth of parents know that it's illegal for children to travel without a car seat or booster seat until the age of 12. Even when children are put in them, 80 per cent are used incorrectly, according to the Transport Research Laboratory – the belts are too slack or not properly adjusted or the actual seat is unsuitable for the youngster's size or age.

"Parents are often in a rush, especially in the mornings," says David Williams, chief executive of GEM Motoring Assist, the road safety and breakdown organisation. "Also responsible is the 'It's never going to happen to me' attitude," he adds. "There's a sense that your child is under your control and only a few feet away from you when they're in the car, so they must be all right."

Then, says Williams, there's the parents who think: "Oh, my son's a big lad, he can skip ahead to a bigger seat than is recommended for him" In fact, your child should go through childhood having had at least three car seats – first, a rear-facing infant carrier (up to 13kg); second, a front-facing full harness seat (8-18kg); and third, a booster seat, with back (15-36kg). Also available from 22kg upwards is a booster cushion that raises the child up so that the belt fits over their shoulder and pelvis.

When buying a new car seat, don't try to judge how safe one is simply by looking at it, cautions Williams. In fact, even those that have won awards from parenting magazines don't always cut the mustard. "The Brio Zento, which isn't cheap at £200, aims to carry children from birth up to 25kg – around seven years old – but it took our child car seat expert 20 minutes to install, and even then it was so bulky that nobody could fit in the seat in front," says David Evans, motoring specialist at Which?. This car seat also achieved poor test results in Which?'s front crash test.

You can't even count on brand names that boast great car seats being consistent. The Volvo Infant car seat, which retails at £197, for example, is recommended as a Which? best buy. But the Volvo Convertible Child seat, which retails at £158, was found by Which? to be unacceptably poor in a front crash and at a high risk of being installed incorrectly.

At least manufacturers are waking up to such criticism. Renolux, for instance, agreed to withdraw one of its seats from the UK, after it performed very badly in a Which? front crash test. The company also agreed to change the design of another model that was hard to fit.

Meanwhile, the Sunshine Kids Monterey seat, whose instructions advised that you could remove the backrest for older children, accepted Which?'s criticism that the seat – when backless – exposes children to serious risk of injury in a side-impact crash. The company changed its instructions, and even the Government is now considering banning backless booster seats and insisting that all under-12s have a booster seat with a back.

Never be led by price, says Duncan Vernon, road safety manager for the Royal Society for the Prevention of Accidents (Rospa). "More expensive is not necessarily better."

You should also avoid buying from any retailer where there aren't experts on hand to make sure that you have the right car seat for the right car for the right child. "You might go along knowing exactly what car seat you want, but unless you try it out in your car – which will have its own particular-sized seats and particular-length seat belts – forget it."

Ask how to use it, adds road safety officer Don Isherwood. "Recently, we were out checking car seats, and in one (not untypical) case the parents put the seat belt on the wrong way round, and the seat would have flipped sideways in a crash, putting the child in serious danger. Even top-performing seats can be compromised if wrongly installed."

Always avoid second-hand car seats and hire car company's car seats, because even the slightest knock in a car can affect a car seat's safety; and, if you've already bought your car seat and want it checked, contact your local road safety officer. They regularly do checks at supermarkets and shopping malls.

Watch out for innovative products aimed at extra safety, adds Williams. "An additional rear-view mirror angled so that you can regularly check on the children's seat belts is invaluable. Then there's the BoostApak – a backpack for children that doubles up as a booster seat and is brilliant in making safety fun." Meanwhile, the baby company Morrck sells baby hoodies which saves parents having to buy thick coats that often loosen harness straps.