Large Radiator hottish at top, warm/cold at bottom

[purplyblue]

The main, very large double radiator in my lounge in my basement flat only get warm, cold at bottom but almost hot at top. Bleeding did not improve this, water that came out was fairly clean. Replaced rad valve, took off thermostat valve top the other side but still the same. All the other rads get hot, but this is the one I need! Have turned others mostly off in the hope that it makes a difference but it doesn't. It was fine and hot last year though. Please help! Thanks

 

[big-all]

probably half full off sludge being the lowest point in the system

 

[purplyblue]

The others are all at the same level and fine, why haven't they got the same problem? If this is the trouble, is there a way of flushing the system without getting rad off the wall? It's big.

 

[big-all]

turn off the valves both ends place a tray basin underneath with towels sheets protecting the wall and carpet
crack the nut open slightly at the bleed end untill you get dripping then slowly open the bleed screw untill you get a safe flow do not remove the bleed screw crack the nut open a bit more iff nessisery

once the radiator is empty try and block the end with a bit off cloth tighten the bleed screw
undo the nut the other end with a tray underneath again when empty try and bung with cloth

the radiator is full off black sludge so be carefull
iff you cant block the holes get help and use your fingers
lift off the wall and invert so the top is at the bottom so the sludge stays in

take outside and hose with a garden hose
the water should be collected and disposed off correctly