How can I fix a Housewarmer 45 Legend Gas Boiler/Fire unit?

My gas boiler consists of one pilot and one main gas valve, a control box, and a thermostat.

When the boiler thermostat is turned on the following happens:
1. The ignitor provides a spark.
2. The pilot valve is turned on.
3. The pilot gas flame is ignited.

After this the ignitor continues to provide sparks but the main gas valve does not energize. The valve is not faulty, it just does not get an output from the control box. The ignition control then shuts down after maybe 20 second. I checked out both valves. Each has a coil resistance of 2 kOhms.

I have replaced the control box with a new one but the result is the same. The only sensing element in the circuit is the thermostat and this only acts to switch supply to the control box.

What I would like to know is, what condition determines the point at which the main gas is to be switched on. How does the control box know if a pilot flame exists before switching on the main valve.

My email address is: richard.daly@cit.ie if you reply by email.