how do i ignite the pilot light on potterton boiler?

You do not say the model of your boiler...

On my Potterton boiler (Profile 100e) when there's a call for heat first the fan runs and an air pressure switch senses that it's running OK - then and only then does the pilot flame come on.

You may have a bad case seal - a bad fan - a bad pressure switch a bad control board or a bad pilot flame gas valve.

A Gas Safe engineer should be able to solve the problem very quickly - be wary of him if he reports more than one failing component - sometimes they guess at your expense until they get it right - you will be paying for his expertise and accuracy in diagnosis. If he reports two bad parts ask him to prove it....

I am not a boiler engineer and am not qualified to give an accurate opinion here - I may be entirely wrong.

Call an engineer...

Peccavi
March 2011