How can I fix my Vaillant Turbomax 282/1E combination boiler?

When I turn on the hot water tap downstairs I cannot get any hot water. On closer investigation the micro-switch attached to the diverter valve is not being fully depressed. I have trying bending the steel lever which converts the movement of the mechanical diverter valve switch; but there was no improvement. I can get hot water upstairs, but only when the tap is turned full on. Can the micro-switch be purchased separately or do I have to puchase the diverter valve? Any ideas as to how to remedy this problem? Many thanks.
Brian Crabb