The pilot light comes on , fan is working but no burner ignition, firing intemittantly

Ian Stainer, November 2008

Call for heat - Fan runs - pressure switch detects fan = OK - opens valve to pilot and ignites it - if pilot hot main gas valve opens. If pilot not hot system shuts off gas.

Your bolier thinks the pilot is not alight or the main gas valve is not opening.

See these URLs

http://www.howtomendit.com/answers.php?id=26383

and (with pictures)

http://kaijaks.co.uk/blog/nick/2008/02/10/dodgy-old-boiler

Peccavi, November 2008