Why does my Potterton Profile Boiler not fire up?

I hope some one out there can help me to diagnose the problem with my Potterton Profile which will not light.

The system provides both CH and hot water, the control pannel/timer appears work OK as the pump in the boiler can be heard running when it is on.

The boiler thermostat knob on the front which goes from 0 - 5 also appears to work, as the system will click off or on when the knob is moved to/from the 0 position. This will aslo cause the pump to cick-in if the timer is set to 'on'. the only thing it does not do is fire up.

Any help appreciated.