Boiler won't ignite?

Hi I have a Worcester 24i RSF (combi natural gas) boiler, I stopped working the other day, it fails to ignite. It couldn't maintain pressure, because the pressure relief valve had gone so I replaced that, I don't know if it was related. But it won't fire up, for heating or hot water. I have gas cooker which still works so I assume it still has gas, the red on indication light illuminates and when switched on I can hear the flume fan. I don't know what componets are in the ignition system, so any possible suggestion would be most welcome. Thanks