Have the rad off the wall, take it outside and flush through with a hosepipe. That's the only sure way, in my view, to get rid of any sludge.
Make sure you turn it upside down, then upright and give it a good purge until the water is running clear. Double-check the bleed valves are clear and working before you refit it. Not unknown for bits of cr@p to lodge themselves around the valves and stop it being bled.
Cheers, I think that's all I'm left with. I'll wait 'til the wife starts whingeing again. Would have done it already if it hadn't have been the big doubler; it is rather heavy!