There are a number of reasons why your domestic hot water is only lukewarm:
COMBI SYSTEM
1) The domestic heat exchanger is all scaled up inside therefore reducing the amount of water being heated up as it flows through.
The remedy is to have it chemically cleaned by a professional or to replace it.
2) The boiler has an undersized gas supply to it , i.e. could be being fed with 15 mm when it should be 22 mm or even 28 mm pipe. There are minimum flow rates which are worked out according to length of pipe runs and how many elbow joints etc.
3) Diverter valve is passing and needs replacing.
4) You have too much flow of water passing through the boiler to be heated. This can be because the hot water supply is piped in 22 mm copper when 15 mm is better for a combi system.
Turning the tap down to reduce flow will result in hotter water.
5) Could be something as simple as turning the hot water knob up a bit on the boiler :D .
CYLINDER HOT WATER
1) Try turning up the cylinder stat which should be set at 60 degrees. Replace if not bringing the water up to temp.
2) If immersion heated, the heating element may be scaled up and needs replacing.