On most boilers the first thing that happens is that the fan runs - you can usually hear it.
When the pressure switch senses that the fan is working OK the pilot ignighter sparks and the pilot gas valve opens and the pilot lights.
When the pilot sensor detects the heat from the pilot flame the main gas valve is opened and the boiler fires up to heat the water.
You do not say which boiler you have - Make & Model.
It is possible to get a fixed-price repair from some large organisations. A reccommendation from a friend or neighbour may be a wise route to follow. All boiler engineers are fine at replacing things but a few are not good at diagnosis and just change things almost randomly until they get a fix - beware these.
Boiler spares are usually rediculously expensive for what they are - a simple fan for instance, of sheet metal and a small electric motor can cost £150 + VAT + Fitting costs. If you have it fixed for under £200 you will have done well.
Good luck...
Peccavi, May 2009
