Rear seat belts - 91' - do i need them?

Im after some seats in the back of my 200tdi so i can ferry mates about without them complaining constantly about how uncomfortable it is...
I will probably just make my own so i dont lose much needed space with bulky bench seats.
However im unsure whether its a legal requirement to have seatbelts in the back of a vehichle this age. Ive heard a few different stories!

Thanks for any useful replies.

Very good question that, I have all the seats taken out of my 90 not that they had seatbelts in the first place, after all whats the use of a belt in a side facing seat? And makes me wonder if i could just put a foam pad either side of the back and legally transport my inbred mates around the country.

Side facing seats don't require seat belts, either for MOT or for carrying passengers on road. Forward facing seats do require seat belts.