I had my block acid dipped etc (and the radiator overhauled) at rebuild time, and even though it looked as clean as there was still lots of sediment that loosened up and dirtied the water up after a short time running, including some bits of scale. So I did another flush and all is good now.
I should have listened to my mechanic though - he suggested putting some stocking material over the end of the top or bottom hose (can't remember which) and all I could find in the mrs' "smalls" drawer was fish nets - ha ha ha - and they are no good as sieves
Peter.