Hi,
Had the same problem myself and found the answer posted on another site. It will involve a small amount of DIY but will save you pounds if it works!
1. Turn off gas supply (red tap underneath boiler) and elctrical supply (fused switch, to side or above/ below boiler) Undo screws on main cover and remove.
2. Undo 4 screws on metal cover facing you (has burner window hole on it).
3. Look to the top of the boiler and you will see the fan and a copper coil.
4. By the copper coil is two electrical connections. Behind and to the side of these connections is a further single connection which should have green/yellow wiring.
5. Carefully pull of the connector and blow inside to remove any dust, wipe the connector pin free of dust/dirt and replace.
6. Try the same for the two connectors by the front near the copper coil.
7. Replace all screws/covers in reverse order.
This worked for me and saved me calling an expensive engineer.
Good luck
Adrian S
December 2009