if your heating is working, there is probably nothing wrong with the timer. They can fail, but usually completely. If it is the timer I know, you want to start by resetting the timer (Indented button on right hand side) you will need to do this with a pencil or similar. Once the screen has returned to '0:00'
you can set up the current time and day.
hold the clock button and press the day button until you see the current day displayed on top. hold the clock button and set the current time with the hour and minute buttons. once this is done you can set up the times.
press timer button once. '1 on' will apear, set this to the first call of the day you wish the heating to come on. press timer again and '1 off' will appear. yo can do this up to 6 times in one day. if you wish to set the same time for every day, press the day button after timer to see the days displayed at the top - setup on mon to sun. when finished press clock to return to the current time.
when finished leave the bottom cursor in the middle 'Auto' position..
if you get it wrong don't worry just have a play you can't harm it..you can always reset and start again