If it is leaking from the spindle, use a spanner to tighten the gland nut. If that proves unsuccessful, undo the nut and wind PTFE tape down into the spindle.
If it is leaking at the coupling to the pipe, tightening the nut gently may be all that is required. If that proves unsuccessful, then drain the radiator. Then undo the cap nut, take off the fitting and replace the olive. Either smear the olive with silicone sealant or wind some PFTE tap around the olive before retightening the cap nut.
If the leak is caused by corrosion, a new radiator needs to be fitted. In an emergency a temporary repair can be made using a plastic resin filler. To replace the radiator, turn off the valves at each end. Remove the cap which holds the lockshield in its position, then fit a key on the top. Turn off the manual valve at the other end. Now the radiator can be removed and replaced. Use wire wool to clean corrosion from the threads of both adapters and blanking plugs and either smear with silicone sealant or wrap PFTE tap around the threads. Check that the wall brackets are secure before replacing the radiator. Connect the valves to their adapters. To help with any future corrosion problems, flush out the existing system with clean water and add a rust inhibitor to the water.
If you need to repair the valve, you will need to drain part, if not all of the system. Check that the replacement valve is the same type as the one being removed. A different kind of valve may not align with the water pipe. Use an adjustable spanner, unscrew the cap nuts connecting the valve to the water pipe, while holding the body of the valve with a wrench. Unscrew the cap nuts holding the valve to the adapter in the end of the radiator. You should now be in a position to remove the valve from the site. Remove the valve adapter from the radiator by unscrewing it. Clean the threads in the end of the radiator.
Wrap PTFE tape round the threads of the new adapter and screw it into the end of the radiator, tightening with a spanner. Next slide the valve cap-nut and a new olive over the end of the pipe and fit the valve. Using a similar method to the dismantling of the old valve - hold the valve with a wrench and align it with the adapter. Then tighten the cap nuts holding the valve to the adapter and the cap nuts holding the valve to the water pipe. The system should now be ready for refilling with water. The site of the fitting should be carefully checked for leaks. If necessary, tighten the cap nuts a little more.
Once the system has been refilled the radiators will need to be ‘bled’, this is done by undoing a screw at the top of the radiator with a special key (usually supplied), this allows any air in the radiator to escape and the radiator to fill completely with water. When bleeding a radiator you should not unscrew the plug completely, just enough for the air to escape. When water starts to some from the screw, the radiator is full and it can be closed. If the whole central heating system has been drained for the repair then all the radiators have to be ‘bled’ to allow them to fill.