Cold at the top is air. It rises to the highest point in the radiator and sits there, and bleeding it releases it. If it comes back within weeks, air is getting in, which is a different and more interesting problem.
Cold at the bottom is sludge. Magnetite settles where the water moves slowest, which is the bottom of the panel, and bleeding does nothing at all for it. One radiator like that is a removal and a flush off the wall. Several, and it is a system question.
Cold all over, with hot pipes at the valves, is the valve. Either a thermostatic head whose pin has stuck down, which is extremely common after a summer of not moving, or a lockshield closed by somebody decorating. The pin test is simple: pull the head off and press the pin.
Radiator Repair is priced after the job has been seen, anywhere in the areas we cover. Send the postcode and a few lines about what it is doing when you contact us, and the answer comes back in writing.