How to fix a leaky radiator - please?

One of my rads is leaking. I don't know the technical part name - but it's at the bottom, opposite side to the TRV. There is the normal bit that you turn off if removing a radiator and then there is an extra silver bit about 1½" high with a bleed screw at the top and then it joins the radiator as normal. It's leaking where the bleed screw head is. We have tried tightening it, but it doesn't help - any help much appreciated.