Simple Guide For Fixing A Leaking Radiator
A leaking valve
First, drain the radiator in question until the water level is below that of the leak. Turn off the lockshield and the supply valves, using towels to catch any water that dribbles out in the process. Using a spanner, undo the union nut, then open the bleed valve to release any trapped water. Using PFTE tape (and plenty of it) wrap up the valve tip before tightening the union nut and reopening the lockshield and bleed valves. Once your radiator has filled again, you can close the bleed valve.
A leaking radiator spindle
A far easier job, a leaking radiator spindle can often be fixed simply by tightening the gland nut with a suitably sized spanner. If that fails to work, you can always undo the nut instead and wrap a bit of PFTE tape around the spindle to make a tighter seal.