If you have a room sensor that is not being used in a room you can use it to control the smart plugs or the single/dual channel receivers.
Step-by-step guide
Create a new room on the app which is a 'radiator room'.
Add a smart plug into it (by using the blue spanner and cog button on the 'add a device' menu on the setup page for that room. Note the ID number of the smart plug is referenced on the 'Choose a datapoint or node to assign to a room' page.
Set the 'Successive minutes of activity to trigger heating' to 2 minutes minimum.
Set the 'Successive minutes of inactivity to stop heating' to as long as you want the smart plug to stay on for.
Set the 'active temperature' on the main page for that room to 28º to make sure that the room is always cold enough to call the smart plug on.
The smart plug in this case thinks it is being used to control an electric heater. If you leave the room on the app in Super Eco mode then it will only turn on the switch when it senses activity, and if you put it into Eco or Comfort mode then it will keep the switch on when it learns that the area of the property is being used.