Potterton boiler not firing up?

I have a floor mounted potterton 11 boiler.
I can light the pilot light having just changed the thermocouple.
Initially the boiler lit up but did not stay running for long.
Now the boiler will not fire up.
I can hear the electrics hum but there is obviously no gas released.
The pilot light however is still alight.
Any ideas would be welcome.