DHW fault on Ferolli Modena 102?

I have a ferolli modena 102 combi and have recently noticed the hot water temperature is not what it used to be. The temp adjuster on front panel makes very little or no difference. I have asked a plumber who said to check thermister and flow switch. I have changed both of these for known good ones yet I still have the fault, any ideas please? One more thing,the water temperature while not hot, is more than luke warm and is constant.