Single Seat Dunebuggy In A Truck With One Double Seat And One Single Seat Do I Need Three Seatbelts?

In a truck with one double seat and one single seat do i need three seatbelts? - single seat dunebuggy

It has dual headquarters, we had only 1-belt. Two ITV have today been given told me that you should have two thedouble seat belts, but without additional anchor?