Worcester 240?

Hi,

My Worcester 240 boiler is playing silly buggers. The pilot light is working but the main burner will not ignite. I can however get it going by turning off the power, turn off gas into boiler, power on again and turn gas on again. The main burner lights okay. Once CH is turned off and the burner goes out, it wont light again unless I go through the process above.

Any ideas?