Bleeding radiator..

[Diasi]

I too held a cloth under the screw.. I did it while the heating was on.. .. I knew it was working because the water hissing out was starting  to get hot..
 I was told once that the heating should be  'off' when bleeding a radiator.. I guess for safety reasons..

The heating should be off & the radiators cold, 
a: for safety
b: to let the air pockets settle as they're not being pumped round the system or even air being drawn into the system.

From what you describe there was magnetite sludge in the radiator which has moved elsewhere in the system & may not cause any issues or may block another radiator at some point.

[Diasi]

We're fortunate insomuch as our central heating has plastic piping & runs on a manifold system & not a ring circuit.

The plastic piping virtually eliminates the magnetite sludge & the manifold system ensures that each radiator gets an equal amount of hot water so they all heat up to the same temperature at the same time.

[Scrumpy]

If you have a cold bottom, sit on the radiator.

I have listened to all that has been said and will not bleed the radiators when water is hot..   I do have insurance cover and someone is coming out in January..  People with greater problems than mine..