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  Step 1: Find the leak

  Hold a bit of tissue against each bit of pipe near the leak for a few seconds.
  
  When the tissue comes away damp, you've found your leak.

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  Step 2: Loosen the union nut

  Put the small pot under the leaky joint.
  Turn off the valves at both ends of the radiator to stop it filling with water.
  Undo the union nut (the one between the feeder pipe and the radiator) using the adjustable spanner.
  Tip: Use the Stillson wrench to hold the feeder pipe in place as you undo the nut, otherwise you could bend the pipe.

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  Step 3: Drain the radiator

  Work the nut completely loose with your fingers.
  Water will start dripping from the radiator as soon as the valve is loosened.
  Let air into the radiator by opening the bleeder valve (the one at the top of the radiator) with the bleeder key.
  Be ready with your bucket as water will gush out of the radiator when you let air in.
  Let the radiator drain down completely.

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  Step 4: Replace the sealant

  Unscrew the union valve completely and remove it from the radiator.
  Scrape away the old gunky PTFE tape from the valve.
  Wind new tape around the threaded area of the valve about 20 times.
  Work the tape right into the thread with your fingers.

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  Step 5: Replace the valve

  Put the union valve back into place and screw it on to the radiator.
  Turn the valves at either end of the radiator back on.
  Leave the bleeder valve open while water fills the radiator.
  If the heat is on, you'll feel the water rising into the radiator. Let it rise until the radiator is full.
  Close the bleeder valve and your radiator should be as good as new.
  Tip: When tightening valves on any plumbing, don't overdo it. Tighten the valve as far as you can by hand and then give it just another half-turn with the wrench.