Vaillant 630 System Boiler/Unistor Cylinder

[nutrino111]

Hi evry-body. Virgin post (honestly). I have a Vaillant 630 System Boiler/Unistor Tank coupled with Acova Steel Column Rads installed approx 2009. Problem I'm having is the hot water is not hot and the boiler stops and starts. Everytime the water is heated the bathroom rad also heats up as previous old system. Vaillant engineer called and said system needs PowerFlush/MagnaClean. Did a PowerFlush and installed MagnaClean but warranty expired. Any advise would b much appreciated. Thanks