It just sounded like the radiator near the thermostat would switch off before the thermostat switches off.
I like the sound of that. I have a couple of remote thermostats, and the downstairs one isn't triggering the system (low battery I assume), so I've taken in upstairs, and kind of doing...
"the temperature put out by the central heating system" Eh? The thermostat simply tells the hearing system whether the desired room temperature has been reached or not. It seems your manual says that your thermostat isn't very accurate.
If the rads are hot, the heating is on and not reaching 18c, so turning the thermostat up won't make any difference. You may have drafts in the house - locate and eliminate.