I presume the fire is turned off? Have you bled all the air out of both radiators?

There’s a damper in our chimney that we can close when the fire is off - it stops draughts. If the fire is on, the bigger problem is generally that the thermostat sees that the room is hot, so shuts off the central heating. If that’s the case you may want to reposition the thermostat

So, either block up the chimney or move your thermostst