Is my Fan faulty?

After having problems with my boiler not supplying any DHW or CH the last couple of days i am of the opinion it is a faulty fan that is the cause and would just like a second "experts" view before forking out for a new one.
The boiler is a Vokera 20-80 R.S. flowmatic. The pump appears to be working fine as does the three port mechanical diverter v/v (thought it might be sticking!). Without any demand on it the fan runs fine with no abnoraml noises or obvious changes in speed. When i turn on the hot water tap or the central heating the fan then comes to a stop, not immediately but after a couple of seconds. Is this normal?? I thought maybe when there is a demand for hot water that the fan should go to high speed rather than stopping. Also by disconnecting the silicone pressure tube to the diff. press. switch and physically blowing into it (and with hot water tap turned on) the main burner then ignites. This is the only time i can get this to ignite, the pilot light is there all the time and i have had no problems at all with that. Any ideas??
Thanks in advance.