Also,
just think of the room thermostat as an on/off switch for the heating. When the heat (in the room) is below the set heat, the heating is on. When the room reaches the set heat, the heating is turned off by the thermostat.
This of course is assuming that the heating is turned on at the timer.
So think of it like this
Boiler -> Timer -> Thermostat -> Radiator (with possible TRV)
Set the timer for the times that you want to heat the house.
Set the thermostat to the heat you want the house to be at (typically 18/19 degrees)
And leave it to do its thing.
If the timer is off - the heat wont go any further
If the timer is on, and the room is at the thermostat setting - heat wont go any further
If the timer is on, and the room temp is lower than thermostat setting - heat goes to the radiators.
The hot water circuit operates independently of the above, but as Noah says, it should be turned on during the same time as the heat is on, for efficiency. It will also have it's own thermostat (I hope) on the tank, so that the water will only reach the set temprature, and will turn off the need for heat into the tank when that temprature is reached (typically around 60 degrees to kill legionaires bactaria.)