ravenheat rsf 820/20?

I have a ravenheat 820/20 C/boiler,The c/heating works fine , but the dom/water is just about warm, I had a new D/Valve fitted but this made no difference. The boiler ignites for both d/w, and c/h. I took the thermostat sensor out of its location with the hot tap open to see if this might make a difference, sadly it didnt. the boiler fired but no hot water, When you adjust the thermometer knob, to high , or, low, you dont hear a click to indicate what the temp is , high or, low, could it be this. or any ideas please. I will try anything within reason.