Do the rads need "bleeding"? (ie they have air in them and you use a radiator key to let the air out). If your rads feel hot at the bottom but cooler at the top, there is air in the top part. Put something underneath to catch the spills and gently turn a hexagonal rad key at the end of the rad at the top. It will start to hiss as the air comes out, and then dribble water. Turn the key back to stop the water escaping.
It should be hot all the way down now - if that was the problem. If it worked do the same with all the rads.
Otherwise, your rads may be silted up, and you will need to deal with the sludge. I can't tell you how to do that, but I think there is something you add to the system to help clear it.
Hez, July 2011
