Vokera syntesi 29. Can anyone help me I am only getting luke warm water ?

Hi I have a Vokera syntesi 29 that is only giving me luke warm water no mater where I turn temp control. Somthing seems to be turning on and off at the boiler as I can hear a clicking when tap is turned on, it is not a constant temp it is warm cold warm cold as tap is runing. Heating side is fine and there is no fault code showing.
Many thanks for looking and any help with this will be very welcome