Potterton Puma-no central heating but hot water ok?

Our Potterton Puma has been messing about over the past ew days. The heating simply will not come on either on timer or constant. The hot water fires up ok though and the pressure is fine. My dad took the cover off to have a look and when he accidentally knocked the lid against the boiler it started working for a while. It's not working now though. HELP!