when i run the bath i have to turn tap down low to get water hot if i run tap at a higher flow it only gets luke warm mainly in winter when water is colder
if it a combi boiler i would say that your domestic hot water heat exchanger has blocked up not allowing enough boiler water to run through it allowing the cold water to warm up. u can remove and try cleaning it or better replace with new but it may block again so i would recommend a powerflush which will clean the system of iron debris prevent further blockages
