Baxi Bahama 100 ?? sensor problem?

Unfortunate owner of Bahama 100 which has had several problems over last few months. Replaced:- Diverter Valve, both boards & flow switches. Also cleaned venturi in the fan which solved the flame cut out at point of ignition. Problem now is that when either central heating or hot water activated the pump runs but there is no ignition. After a few seconds the pump swiches off. In central heating mode this cycle will eventually repeat although after some time. SIT 504 NAC gas valve has never been any problem. Could the problem be the temp. sensors on the main pipe from the heat exchanger one with red the other black wiring? Can't think of anything else other than a Worcester!!! Any ideas would be truly gratefully accepted. Thanks.