This is all very simple, maybe consider a thermal store instead of a standard hot water cylinder, as for controls please see the following link to save me alot of typing. http://www.termoventiler.se/default.asp?webb_ID=110&webbsida_ID=56
A thermal store is used to store energy in the form of hotwater (80 deg C), this hotwater is then be used to to provide space heating and hot water on demand.
The DHW (hot taps) is supplied by passing cold water direct from the mains through a coil in the top of the thermal store, this heats the water instantly by conduction. For saftey the tempreture of this water is then normally regulated by a thermostatic blending valve before it arrives at the taps.
The hotwater in the thermal store can be pumped directly around the underfloor heating circuit or via a coil in the store like the hotwater, either way you should have thermostatic blending valve between the store and floor to prevent damage caused by high temps.
A thermal store should be sized with both the demand and the recharge interval in mind, the bigger the store the more energy it can hold between boiler firings. For example if configured correctly a 500 litre store heated to 80 deg C could hold around 23 kW of usable energy.
all the best