Originally Posted by 
hkstroud
				 
			Assuming that you are in EU or UK.
In the US the mains supply provide water to the water heater and to all taps, therefore water pressure is always the equal.  Is that the same for you?
In the US most currently sold tub and shower valves are single handle.  All currently sold, single lever tub and shower valves have a pressure regulating valve.  This pressure regulating valve is sometimes referred to as a temperature balancing valve.  It is not really a temperature regulating valve but regulates temperature by balancing the water pressure.  It is adjustable.
Some tub and shower valves have integral stop valves.  These are accessible only with the escutcheon removed.  If your new tub and shower valve has integral stop valves, are they both fully open?
If you new tub and shower valve has a pressure regulating valve, have you tried adjusting.