Myson alpha 30/50 B boiler not firing up?

Hi All,
The alpha 30/50B boiler was manufactured in 1990 and the pilot light is working.When the unit is switched on there is power going to the main solenoid which is switching correctly.It appears to be that gas is getting to the pilot side of the main gas valve but is not getting through to the burner valve.
Is there anything that can be done to check out the valve or do I need a new valve (approximate cost if anyone knows please??).
I was hoping to get this boiler going for another little while until I have the time to get a new one fitted.

Thanks,
OllieC