my pilot light has gone out on our Greenstar 30CDI, how do I re-light it?

My heating has been working intermittently this afternoon after working completely fine for that last couple of years. The mains blue light is still on, but the pilot light is now not lit on the control panel and the boiler is naturally now not heating our radiators. The pressure is at 1.5 bar, so what do you think could be wrong and more importantly what can I do to rectify the situation?