Funnily enough, there's a question about the legal requirements for rear seat-belts in the June issue of Practical Motorhome (I often suspect they get their ideas from this site!)
Their reply is similar to some members comments on this thread....
Basically, if the vehicle was registered on or after 20th October 2007, it is illegal to retro-fit seatbelts to inward-facing seats. EC directives state that they are unsafe, and all passengers should be in forward-facing or rearward-facing seats with three-point belts and head restraints.
If the vehicle was registered before that date, then there are "few legal implications for carrying passengers in the rear of the 'van. They can travel in motorhomes without rear seatbelts, but if it does have belts of any kind fitted, then they should be worn".
Although it's not a legal requirement in the older vans (registered prior to 20/10/2007), they would still advise that passengers should only be carried in forward-facing, or rearward-facing seats, and the seats should at least have lap-belts, but preferably three-point seatbelts fitted.
(But even in cases where it's not illegal to have rear passengers without seatbelts, there is still the possibility of being charged with carrying an unsafe load, which I guess would be more likely in the event of accident/injury/
insurance claims?)