Do you have to double thread the seat belt to ensure the thickness is suitable for the safety belt gripper? The guy at Halfords suspected this was how it should be fitted, but I can't locate the original manual. When the seatbelt was used singly - the seatbelt gripper did not grip the seatbelt, it slipped around causing movement of the seat. When the man doubled up the seatbelt, it did work sufficiently. He asked me to check if this was the correct procedure.