Close off the rad' valves. Open the stop cap at the top of the rad' above the bit that's leaking. Get a raw egg. Crack it into a saucer. Take a good teaspoon full of the thick part of the white (the clear stuff) from around the yolk. Pour it all into the rad' through the open stop cap using a bit of folded paper as a chute. Wait. It wont take long. Just wait. If it hasn't stopped the leak within half an hour, you can up here and shout at me. When it's stopped. Put the stop cap back in place and tighten fully.
Now. Go put the frying pan on a medium heat on the front burner of the stove. Put a knob of butter in the pan. Take the saucer with the remaining egg on it and pour it into the melted butter in the pan. While it's cooking away, put the kettle on and make a cuppa'. Butter a couple of slices of thick bread. Go to the frying pan and use something to lift the cooked egg out and lay on the bread. Slap the other bit bread on the top of it. Pick up you tea. Go sit in the nearest big comfy chair. And Relax.
You see, it took longer to make the tea and sarnies, than it did to fix the radiator. :-))