combiboiler minic24 hot water problems?

i have a combi mini c24 heating/hot water boiler and when i turn the taps on the hot water does not run constantly i have to run the tap very slow to gain hot water it is almost impossible to run a hot bath when water leaves tap it keeps changing from warm to cold
engineer fitted me some type of valve/thermo ???
but still having problems please help
(in need of a hot bath)