apollo myson not firing up, pilot light still on?

Hi there, Can anyone give me any tips to go on with my apollo myson boiler which stopped working last night. The pilot light is still on and when you try to switch it on there is a definate click followed by a low buzz noise, but no firing? I had a problem a while back which urned out to be the 3 way valve and I have had that sitting at manually open for approx 2-3 months now. This problem seems different though as the system doesn't fire up at all where as before it would fire up then switch off after a short while.

many thanks

Colin.
January 2008