I'm answering my own question here as I want to share this information.

I have a potterton boiler that fired up and then cut out, pump starts, fan starts, gas on, ignites, cuts out the cycle repeats about 5 times before the control logic gives up.

If this happens to you try draining the condensate trap and drying it (i used a hair dryer)

In my case the boiler has a flame detection electrode that is also linked by the same wire to the condensate trap. If this overflows or gets damp on top the detecton wire is shorted to earth by the water.

The boiler thinks either there is no flame and cuts gas out or that there is a problem with the water level and cuts out.

I also read that you should clean the detection electrode with wire wool if needed, this may be your problem instead.

I hope this may help someone someday!

Phil, January 2009

Thanks Phil. Your solution saved Christmas firm being a very chilly affair. Condensate trap was full of water as outlet had frozen. Managed to defrost and dry out with hair drier and boiler worked again.

Mark, December 2010

Phil
You are a legend, have a virtual Pint.
My potterton started doing exactly as you described. The chap that serviced it last, for the previous owners of my house, diagnosed a circuit board problem and recommended a potterton specialist for £300.
I tried the wire wool trick and it fired first time.
Thanks again

Paul, January 2009