The mechanism comprises a number of spacing and transporting elements (57) travelling along a straight section of a conveyor comprising upper (25) and lower (26) endless guide and drive chains. The elements (57) are connected by supports (32). It has a third endless guide and drive chain (31) situated beneath the other two and connected to the supports by levers (50). Each lever (50) is able to rotate relative to the third guide and drive chain (31) about one vertical axis at the end of the lever and another in line with the support to which it is attached. The first two chains are design to pass round control (24) and return (30) heads, while the third passes round two separate return sprockets (19, 20). In each case the chains can be replaced by belts passing round pulleys instead of sprockets.