timer control for C/H?

We have just moved into a house with a Potterton Boiler Performa 28 we have no manual, the setting device is not moving when we set the dial to the times we want, it does not tell us how to have the setting to go off once or twice a day, as far as we can see it has a settingf for 1 at the top nothing in the middle and 0 at the bottom when trying to turn the dial it is very stiff and difficult to move and does not move as it should when when switch on to C/H. we have looked at the instructions we have but there is no info on how to set the timer that part of the manual is blank.