Cold at the top and warm at the bottom means air, and bleeding it takes two minutes. Cold at the bottom and warm at the top means sludge, which is a different and bigger job.
- Turn the heating off and let the system cool.
- Start with the radiator furthest from the boiler and work back.
- Open the bleed valve at the top with a radiator key, with a cloth and a cup underneath. Hissing is air escaping.
- Close it the moment water comes out, which is the point of the whole exercise.
- Check the boiler pressure afterwards and top up if it has dropped below the marked band. Bleeding always drops it.
Cold at the bottom is not air and bleeding will not fix it. That is sludge, which needs a power flush or the radiator taking off and flushing, and continuing to run the system that way damages the pump and the boiler.
Air rises to the top of a radiator and blocks the hot water from filling it. Sludge, an iron oxide slurry from corrosion inside the system, sinks to the bottom and does the same thing from the other direction.
Gas work requires Gas Safe registration by law and most electrical work is notifiable. Where a tip says to call somebody, that is the advice. Full disclaimer.