My old Stag had inertia seat belts retro fitted in the the back, which failed the mot due to the design of the rear seat and roof storage compartment not being a strong enough structure to offer any support in an accident When asked how i go about using the car with the kids in the back, he said that since seat belts where never fitted in the back originally then you don't need them As baybirmingham says children must be 3 or older to travel unrestrained in any vehicle
I dont have them at the moment but will be fitting non inertia lapbelts in the rear in the future I did once find a child seat that could be secured only using a lap belt but since my old computer died ive lost the link
Here's a link to an article that shows the kids lap belt seat http://www.quickfitsbs.com/PDFs/vw_camper.pdf
