These normally sell for £9.99, Argos have them at £3.99 which is a good price. I think it also featured on the Dragon's Den a while ago.
The Autosafe Seat Belt Height Adjuster helps protect your child by aligning the seat belt away from your child's neck.
* Easy to fit.
* Autosafe height adjuster has been crash tested to ECE44/03 and is proven to reduce injuries and significantly improve child safety.
* Adjust the height of the seat belt strap so that it rests just above the shoulder to prevent the seat belt from rubbing the child's neck and to ensure that children do not place their arm over the belt.
* For use with booster cushions which have no height adjuster strap.
* Can be used for children over 135cm for additional comfort/safety and in cars where a booster seat will not fit or is not practical to use.