Vaillant ecoTEC Plus no hot water, but heating works fine !!?

Hi, hope you can help. My heating is working fine however I'm not getting any hot water when turning on the tap(s). With the hot water tap turned on the boiler recognises this and the tap symbol appears, the bolier starts to work and I can hear the pump, flame symbol, gas valve and internal pump symbols are all on however still get no hot water. I have tried resetting the unit and also switched the heating off and just run the hot water/warm start, still no joy. There are no error codes. Please help as I need to have a shower :-)