Thermostat on radiators - how do they work?

[Bosso]

I have gas heating in my flat and the major radiators have thermostats on them (the dial that goes from 1 to 5). To me I thought that by lowering the knob from 5 to say 3 should drop the temperature of the radiator but this doesn't appear to be the case.
My sitting room radiator is quite big and is always piping but the temperature does not change using this knob as when I get to about three the radiator does not heat up. Change the thermostat twice thinking it may be faulty but still no joy.
I am just wondering what the purpose of it is if it doesn't make a noticable difference. Am I doing something wrong? Would it make a difference if I restricted the amount of water going into the radiator by closing the valve a little, not the proper term i'm sure but what I refer to is the opposite end of the radiator under the small white cap.

As you can see I'm not a plumber and am at a lose to explaining my problem professionally so hope you kind of get some picture as to what I am referring to.

Thanks in advance for any help.

 

[leakydave]

The TRV wont change the temperature of the water reaching the radiator. What it will do, if working, is react to the room temperature and close, depending on what setting you have put it on, and the radiator will stop getting hotter and start to cool, cooling the room. Eventually, as temp drops, the TRV will open and so on...