My backboiler is no longer heating water?

I have a dovedale 4 BBU backboiler. It worked well (always has) today, and then this evening when I turned it on again to boost the temperature of the house, it no longer worked. The ignition is clicking over and the pilot light is on but it won't ignite. (or heat the house!) the fire front works perfectly. Its a very windly day so I considered it might the Flue blockage safety device - however a the fire front is working that doesn't make sense. The reset button is not alight (tried it anyway). Any suggestions?