Hot water Please help

[sak]

I have Biasi M110.32SM/C boiler fitted, it was working fine last 2/3 years,recently divertor service kit was replaced,because every time i turned the hot water heating was kicking in,so i rang the company and was told that it was divertor valve.that problem was sorted,but now every time i run the hot water boiler comes on,but its not giving me hot water.water is either luke warm or some time gets hot,but most of time is cold. i also changed the membrane kit. Any idea??

thanks