Took rad off as I am decorating room.
Thought it an opportune moment to flush it out in garden.
Clear water and inhibitor came out when I drained it in the room prior
to taking it off the wall.
Flushing it with garden hosepipe shoved out loads of black sludge ..
and I mean loads ... yuk.
Second rad from same room, not so bad.
Now thinking to do them all as every room is being decorated.

Would the upstairs ones be more likely to be cleaner than the
downstairs ones ?

Mike P