Vaillant Turbomax plus 828e combination boiler?

Hot radiators, but luke warm water from the 'hot' (bath etc) tap elsewhere in the house although the water temperature from the taps rises slighty if I turn down the pressure of the (bath) tap so the water just trickles out. If I turn the 'hot' (bath) tap on full the water goes back to barely luke warm again. The boiler is just over two years old. Any suggestions please to fix the problem.