How can I fix my 99 Fiesta starting problem?

I have a Fiesta (year 1999) from new with 62K on the clock,have had no problems until recently,when it failed to start.
It actually turned over a few times but chugged a lot and eventually it just turned over but no ignition.
Got it to a Main dealer and they said they could find nothing and it started straight away for them.
Today it has had the same problem within a week.
I cannot hear the fuel pump running but am wondering if it is an electrical fault/damaged relay/fuel pump problem/or another ignition problem.