If your radiator has cool spots at the bottom then chances are there is sludge present. This will slow and ultimatley block the heat travelling around the radiator. Please allow the radiator to cool before working on it and we advise that two people do this job.
Place down dustsheets where you are working so you don't make a mess.
Turn off both radiator valves either side of the radiator by turning them clockwise. You may need to remove the top of the lockshield valve (thats the one that either won't move or is just spinning) and use a pair of pliers to turn it shut.
Drain the water out of the radiator by connecting a hose pipe to your radiator valve drain off point. If your radiator valves don't have a drain off point then loosen the nuts to the radiator and drain off the water into a suitable container.
Once the radiator is completely empty, undo the nuts holding the radiator to the radiator valves. Now slowly and carefully lift the radiator up off of the bracket making sure to cover the open ends with your thumb or something else that will stop any drips coming out.
Remove the radiator outside to a drain and pour away any water left in the radiator. Now connect a hose pipe to the cold water, either an inside or outside tap and connect the other side to one of the radiator tailpeices (the bit with the nut on that used to be connected to the valve). Make sure both sides of the hose have a jubilee clip securing them.
Point the tailpeice that isn't attached to the hose pipe to the drain. This will prevent the sludge going everywhere. Slowly turn on the tap and allow the water to run through the radiator.
Once the water starts to clear turn off the tap and disconnect the hose from the radiator. Pour a little of the water out of the radiator but leave half. Now cover both ends by thumbing it or by another means and tilt the radiator back and forth a few times.
Empty the water and if not clear repeat again with the hose and tilting steps until clear.
Once the water is clear again empty the radiator and reconnect it to the bracket and valves. Make sure to tighten up the nuts.
Turn both valves back on and bleed the radiator, test that it works and check for leaks.
Get someone to help you with this, there is a bit of lifting involved and it can be tricky.
Allow the radiator to cool before you start work.
Repeat this procedure with all the affected radiators.
Where possible pre-treat the system with a desludging chemical (not on primatic systems).
Where possible add an inhibitor chemical to the system after the flush,(not on primatic systems)this will stop the system sludging up again.