Standard procedure is to remove the rads and take them outside and give them a really good wash out with a hosepipe. While each rad is off, flush its supply pipes by opening each valve in turn, and letting water flow into a bowl or bucket until it runs clear - making sure you've got plenty of water in the F&E tank. Then drain and re-fill the whole system, adding the requisite amount of inhibitor to discourage further sludge accummulation. Try not to spill any sludge on the carpets - it's pretty indelible!
You *can* get chemicals which are supposed to dissolve the sludge and allow it to be drained out without dismantling, but they don't work if it's badly sludged up.
The only other alternative is a power flush - but most firms will charge several hundred quid for doing this. I believe you can hire the kit for less though, and DIY.
Probably not sludge if the middle is cold - that would be a hell of a lot of sludge. Check if you have flow by feeling both pipes which should be both hot. If not, a lot more info is needed about your system to give advice. See FAQ for general CH info and balancing info - if that is what the problem is.
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