My Potterton Envoy 40 is showing an error P. What could be the fault?

My Potterton Envoy 40 failed today where it was just indicating a "0" when in the on low position.
After switching off then on again the display shows it flashing between "L" and "P" constantly until it then goes back to "0".
The boiler never starts up as the "P" keeps flashing up before it can start up.
What could be the cause of this and how can I fix this?