Boiler help! Potterton boiler and immersion tank?

I have a Potterton boiler and hot water tank in my new house but I can't get any hot water. The boiler and tank are both on timers and are set to come on in the morning and the evening. It has worked once so I know there is hot water but hasn't happened since. How do I set the timers to get it to work properly? Does it make any difference that the timer has one setting for heating system and a setting for water heating?