I have Valliant Turbo bolier too with a similar problem.....
When I turn a hot tap on the bolier does not fire up, the water just runs cold. If I get the bolier to fire up for the CH heating the hot water will run hot. If I turn the CH off or the temp down on the bolier itself the burner stops and the hot water soon runs cold. I have to keep the boiler on its CH and tap setting to get any hot water, it just sits there dormant if its only on its tap setting.
Does anyone think this is the coomin divertor valve problem too?

Rob Harden, August 2008

could be the diaphram inside the diverter valve if this is split it will not lift the pin above it and this means the micro switch for hot water operation is not being activated the heating is switched seperatley my advice is to get corgi and valiant approved engineer im a plumber and i would not mess with valiant combi unless you know what you are doing they are very complicated units and models differ in design

steve the plumber, October 2008