Hot water problem with my combi boiler

I've got a baffling problem with my Vokera combi boiler (it is a 20-80 RS Flowmatic, which sounds like something out of Wallace and Gromit to me).
When I turn on the hot tap the boiler doesn't light unless it is already warm. Typically, the only way I can get hot water is by turning the central heating on first. Once the boiler has lit and I have the hot tap running, I can turn off the central heating and the hot water will carry on flowing all day. Interestingly, the trick of turning the heating on doesn't work if the hot tap is already running - in fact it stops the heating from coming on too.
I have verified that the flow sensor is working (I can see it make the switch contact and I hear the resulting relay clicks). However, at that point, the fan in the flue should speed up - it doesn't. When the boiler is warm and the hot water system is working as it should, I can hear the flue fan speedup a few seconds after the tap is opened. It only takes a couple of seconds for the boiler to light once the flue fan is going.
Any ideas?
Thanks
- Andrew
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Andrew Bainbridge wrote:

If there is 230V on the fan and it does not (always) start then replace the fan. Else replace the PCB.
Both available I beleive from cetltd.com
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Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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