HOW TO SET TIMER ON WORCESTER 24CDI BOILER?

I HAVE RECENTLY MOVED INTO A NEW HOUSE WITH THE ABOVE BOILER/HEATING SYSTEM PRE-INSTALLED - I AM HAVING TROUBLE SETTING THE TIMER FOR CENTRAL HEATING - IT IS A 24-HOUR TIMER MUCH LIKE THE PLUG-IN LIGHT TIMERS WITH 15 MINUTE SEGMENTS... I HAVE SET THE TIME CORRECTLY AND TRIED FLICKING THE SWITCH TO AUTO AND SELECTING THE TIME HOWEVER HEATING NOT COMING ON??? IS THERE ANYTHING I AM NOT DOING!? SHOULD THE HEATING BE SET TO ON WITH THE TIMES SELECTED WHEN I WANT HEATING ON/OFF?