Hmm yes there should be something even if they are sludged,even cool to the touch,
I dont really buy into the whole power flush stuff tbh,I installed my heating about 20yrs ago,before there was this whole "thing" about inhibitors etc,and its still going strong as it was the first day it was installed,newer boiler now but no flushing etc etc,
Is it a combi boiler or a heating boiler?
If it was me,I would find out where the heat stops and see if I could crack a fitting near there to see if there is any air trapped,
sometimes years ago there used to be a valve put on the return pipe at the hot press so you could adjust how fast the water flowed to the cylinder first before going to the rads,
During the summer you would open this fully as all you wanted was hot water,then when winter came you would turn it back so as to balance the ratio of flow between the cylinder and the rads,
Dont suppose you have this valve and have forgotten to adjust it?