That would indicate the thermostat is behaving as expected. The command from the app is sent to the server, from the server to the display and from the display to the Heat Link. As the heating is turned on and off the relay inside of the Heat Link makes an audible click. That generally indicates that something else is causing the behaviour.
However, since you mentioned it never stays on for more than 10 minutes let us also do this. Like before, go next to it and turn it on. Listen for the click and wait for the boiler to go off. At that point check in the Nest app if it still shows as on and make sure to listen for any click from the Heat Link.
If the boiler turns off and the thermostat shows as on and there's no click, at that point you can turn the temperature down to see if you get a click from the Heat Link, we can be pretty sure that something else and not the thermostat is causing the behaviour.
