Solved. Someone on another forum suggested pressing the AD button, plus the arrow button to the right of the OK button. Thanks to all who tried to help x
When I press mine is says 'off' but if I click on it it changes to 'on' and then if I click again it changes to 'off'. The TV upstairs gives you both options in a drop-down menu. So obviously all different; you need to play about with it, experiment.
There is a button on our remote at bottom right which says AD.
Sometimes ours seems to switch itself on and you then realise there is a voice telling you that 'Chris is sitting on a bench showing a graph to Michaela' or 'Martin is walking across a field' or whatever!