how can i fix my worcester 28cdi boiler?

Hello

My boiler has recently stopped working. The boiler is still getting gas and electricity to it, all the lights are coming on and I can see the pilot light which comes on when I turn on the tap for the hot water. However when I turn on the hot water tap, the boiler makes a clattering noise and steam starts coming out of top so I just turn it off, I don't want hot water to explode everywhere! Is there anything simple I can do myself to fix this or do I need to get an engineer to have a look see?