Potterton Puma 80 runs constantly then trips out?

Following on from a problem with a sticking micro switch, the pilot light went out on my Potterton Puma 80.

I have re lit the pilot light but when I turn the system from standby to on the burners fire up even though the heating is off. It then runs for a short period before it shuts itself down.

Can anyone answer why the burners are coming on and what I need to do to stop this problem. Also what do I need to do to resolve the issue of the boiler shutting itself down.

Thanks for your help.