Daft question have you tried tighten it up, using a spanner?
If so and still leaking, depending which side of valve is leaking, If it's leaking from the joint between valve and radiator you need to close valves at each end of radiator, find something to catch the water in and undo nut at leak, get some ptf tape (from DIY shop or Plumbers Merchant) wrap a few turns around thread on radiator that you have just undone, then tighten nut back up so tape is being threaded over creating a seal between inside of nut and threaded pipe.
If leak is from tail pipe to valve you need to drain system down and do the same as above with thread on valve and ptf tape.
Then turn valves back on and fill system back up to about 1.5 bars of pressure.
and unless the pipe is damaged you should have cracked it!
Hope this does trick
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