Myson Apollo 15/30 intermittent fault?

Can anyone help ?- This is one is baffling me! Boiler stays working for about fortnight & the pilot light always stays lit . The solenoid moves/clicks when controls call for heat , but boiler does not ignite. I've tried both & high & low heat settings- I replaced the thermocouple last time as it may be a poss cause. Am I now down to replacing the Honeywell gas valve or is there another possible cause apart from igniter being blocked as someone else suggested?