strange radiator heat pattern?

the Radiator (normal Stelrad - nothing fancy) is hot at the top and cold at the bottom, the inlet side pipe is red hot, the outlet side is cool. All the other radiators in the house are OK - this one is in the middle of the system and the radiators before and after it are OK. We've been told a system flush should solve the problem.

Could it be a knackered radiator with an internal blockage or a blockage in the outlet pipe and how might I prove/disprove that?

thanks.