Worcester 28CDI - air flow problem?

We have recently moved into a house with a Worcester 28CDI boiler. I've downloaded the instruction manual from the internet and for past few months everything working fine. However, when a hot water tap is turned on we get hot water for a few minutes and then the airflow fault light flashes and no more hot water. The light resets itself after a few minutes and we can get hot water again for a few more minutes until the light starts flashing again. I've checked the flue and all seems clear, but is there anything else that I can check simply and easily before calling out an engineer?

Wendy
October 2008