Step One: Location
To find the source of the leak, dry the radiator with an old towel or kitchen paper. You will need to observe the body of the radiator, the valves, the pipework, and any nuts. The area that becomes wet first is the most likely root of the leak.
Step Two: Protection
Use old tea towels, plastic bowls, or dishes to catch and soak up the leaking water. This will protect your carpet or tiles from further damage. Make sure to do this before you begin further investigations as you may accidentally create a sudden deluge. Also ensure you know where to turn off your mains water supply.
Step Three: Check the Valve
The most common cause of radiator leaks is a faulty gland nut connection in the valve. You may see a patch of water on the floor at the bottom of the pipe that leads up to the rubber topped valve. This is most likely to have run down the valve pipe from the nut inside the valve.
Unscrew the screw holding on the rubber cover and remove both. If there is a square stop on the spindle remove this. Remove the nut with a spanner. If the valve has been leaking you will see that these areas are wet.
You will now need to repack under the nut with a rolled-up piece of PTFE tape. Rip off an inch or two of the tape and let it roll into a string. Wrap this around the spindle and push it down below the thread.
Replace the nut, tightening it as it was before. Replace the square stop, the rubber cover, and the screw. You should be able to turn the rubber stopper without too much resistance or too much ease.
Observe to see if the leak continues. You may need to add more PTFE tape packing to stop the leak. Repeat the above steps to do this.
Step Four: Check the Main Body
Sometimes a pinhole can appear in the main panel of the radiator. This can be resolved in the short term but, eventually, you will need to replace the radiator. Make sure you use a reputable radiator supplier, such as Stelrad, and select a radiator which allows you to use the current pipework configuration.
A plastic resin sealant can be added to the system as a temporary solution. However, you need to know whether you have an open or closed heating system to assess which sealants are suitable. Again, this is only a temporary solution.
Step Five: Check the Radiator Coupling
The coupling is where the pipe from the radiator connects with the valve. It may be that the nut simply needs tightening. Use a spanner to tighten the nut but be sure not to overtighten. If this does not stop the leak in this area, the next step is to remove the olive (small ring) inside the coupling and pack with PFET tape. Unfortunately, this requires a full drainage of the system. At this point you will need to watch a video about the processes involved to decide if you are confident enough at DIY or would prefer to call a plumber.