How can I fix a Potterton Prima F Boiler?

I have a Prima F Boiler. The problem I am having is when I call for either hot water or central heating the fan kicks in for around 10 secs then stops. 20 Seconds or so later it starts again. This action continues but with no flames, resulting in no hot water.

I have spoken to my friend he believes that it may be the flow switch. Can anyone else give me light o my problem.