Baxi bahama switches between HW and CH every few seconds

- Baxi Bahama 100 - Fires up fine and stays fired. - Electrnic relay switch keeps switching routing between CH and HW. - So I can't run hot water for more than 5 seconds at a time.
My baxi bahama boiler has developed a new and exciting problem. Like many, I have have the flow switch replaced about 6 months ago, when the water was cutting in an out all the time and then gave up completely. Now I have a new problem which I haven't found mentioned in any groups. When I turn on the ht water the boiler fires up and stays fired, no problem there. But the water runs hot -cold-hot-cold-hot etc every 6 or 7 seconds. I can hear an electronic relay clicking each time it does it. So the boiler is firing but the hot water is not getting through. About half an hour after running a bowl of water for washing up (which took some time!) i was upstairs and happened to lean against a radiator which was warm. So I think that this little relay is switching me back and forth between central heating and hot water without me asking, and the thermostat on 0, and the boiler switched to the hot water only position. Besides which the Baxi should override CH with HW anyway.
Any ideas!
Last time I had a problem repaired we went through a lot of hassle ordering new circuit boards etc before it turned out to be the dodgy flow switch, and then had a bit a fun time trying to convince the central heating company to not charge for everything they had once thought of doing.

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