why does the pump keep running when I turn the hot tap off?

My Vaillant combi boiler VCWE 240 works fine with the heating, but when I use the hot water, it doesn't switch the pump off when I turn the tap off. I've had Vaillant techs out & they just fiddled with what I think was the pressure switch, & said I didn't need a new part. But after the 3 months gaurantee period was up we're back to the same senario of the pump keep running on. Does anyone know what the exact part I need is called & we'd get someone else to fit it for us.