If, what I have been told about the Spanish MOT laws are correct wouldn't putting in extra seats in any vehicle that didn't come factory fitted with these seats break the Spanish ITV laws / rules?
In the case of the Kango and Berlingo anyone fitting one extra rear seat to make it an 8 seater needs to sober up, even these models that carry invalid chairs / motorised or otherwise are vastly modified which can included the floor pan, seat belt fixings etc, and even then it should be done by an approved registered fitting service, and registered as such.
The Kango and Berlingo are both super light weight cars as they are, putting in extra seats, plus possibly 3 x 10 stone extra people, 8 in all, leaving the steering, brakes, tyres and suspension as a standard car...Not far short of an accident waiting to happen.