Randall 102 not turning ?

Hi. I have a Randall 102 central heating control in my house and it has stopped turning on its own. You can turn it by hand to get the Hot Water or Heating to come on but it doesnt turn on its own so that the heating or hot water comes on using the timer. I thought it may have come off the Cog which makes it turn as you can also turn it slightly backwards but then would i stil beable to turn it by hand easily if it had come off the cog? All the answers i have found refer to removing a screw from the underside but i can not get to the underside as there is another control right underneath it altho there doesnt seem to be anything on the bottom of the randal 102 as i can push my finger up inside it so i assumed that the bottom has already been removed but i can not get the randall 102 to slide up or off?