a flange thereon rotation of spool 54 by shaft 38 will cause lifting of barrier member 22 by either cable 56 being unwrapped from spool 54 whilst cable 58 is wrapped onto spool 54 or vice versa spool 54 is located very close to circular arcuate section 30 and under tension to ensure that cables 56 58 do not lift from their peripheral contact therewith and maximize the mechanical advantage obtained from this positioning fig6 shows a similar embodiment to that of fig5 but there is only one cable 64 which is secured at each end of circular arcuate section 30 and to spool 54 rotation of spool 54 will lengthen or shorten the opposing sides of cable 64 from spool 54 depending on the direction of rotation fig7 shows a similar embodiment to that of fig5 but there is only one cable 66 which is secured at each end of circular arcuate section 30 cable 66 is looped around spool 54 under tension for a couple of turns to provide sufficient frictional force to avoid slippage of cable 66 on spool 54 fig8 shows a similar embodiment to that of fig7 except that spool 54 is replaced by a pulley 68 which is driven by.