Boiler turned off, tie up the ballcock, then...
Try to bail out/clean out the header tank, to prevent debris going down. See if there are any drain cocks in the lower parts of the pipework - it all becomes more difficult without them. Drain, fill, drain, fill. Depending on how bad, some might suggest taking individual radiators outdoors, for a more thorough flush through.
Once you are satisfied it is clean, refill, but ensure you add inhibitor to the fill.
