Rules will be the same for booster seat use in mh as it is in cars and there is an advisory age/height guidance for a child deemed 'safe' to use a booster seat. See Mothercare for practical guidance on regulations before you buy.
The key difference between the child safety seat and the booster seat of course is that the former has its own full harness for the child which is built into the seat itself and the whole lot then attaches to the vehicle, whilst the booster seat has no harness of its own but simply relies on the vehicle seat belt to hold both the child and the booster seat to the vehicle.
The key thing about safe use of booster seat is that the vehicle's own seat belt must safely 'trap' the child against the booster seat and effectively 'sandwich' the booster seat between the child's body and the vehicle's own seat. In effect the booster seat must raise the child's body into a position such that the vehicle's own seat belt operates effectively around the child. Things like shape and size of booster seat cushions, headrest, side protection structures and seat belt guides are vital in 'fitting' the child to the seat so essential to try them out together first. Base of booster seat also important as not all will sit happily on all vehicle seat cushions so try before you buy again important. Mothercare and Halfords both good at providing this service.
Mothercare (no connections with them!) have got a great offer, in store and on line, on their own brand booster seat at the moment (£40 from £60) which is made by Britax and is exactly the same as the much more expensive Britax seat. Visit to local store highly recommended
