It was me that suggested using sludge remover to you, but I do agree that if you are happy with the prospect of removing rads off the wall then it's a better solution.
You get the rads off the wall by unscrewing the 2 large nuts each end at the bottom. This allows enough leeway/movement from the pipes to lift the rad upwards off the wall brackets.
Its a lot simpler to drain down the system and do all rads together, one at a time. Drain down by finding the drain point at the lowest point in the pipe network. In a 4 bed house there may as much as 20 gallons (100 litres) of water in the sealed system. Some houses don't have a drain point low down - in which case you have to drain off using one of the downstairs rads. I do this by closing down both lockshield valves - one each end - on the first rad. This stops the water in the system from draining out and you only have to worry about the water in the rad you are working on. Then unscrew one end gradually and catch the water (having released the air bleed valve at the top of the rad to allow air in as the water level falls). You can turn the nut tighter again to stop the water flow to allow you to empty the receptacle. That way you are in control of what's happening. Eventually the flow stops and you can remove the other end nut. Beware - bottom of the rad is where the black gung is and more water/gung will come out as soon as the rad is lifted slightly at one end! Use old sheets etc. to be safe with carpets.
Having got one rad off, you can drain the rest of the system by releasing gently the lockshield and catching the water. Do both ends in turn.
Remove each rad in turn, take outside to wash gunk out and replace.