Gas fire will not light?

Hi

I have a Portway Pride Fan Flue Convector Gas Fire that will not light. Although the pilot light works fine and stays lit; when I turn the knob to On and then press the switch to light the fire the fan comes on but the fire does not kick in and light. This has happenned before and they replaced something (nor sure what) and was quite expensive. Anybody know what the problem may be.

Thanks in advance