Paul,
Before you use any any proprietory flushing agent, I suggest you remove the drain plug on the left of the block, open the rad drain tap and run water through the system, via a hose into the rad filler cap, until it runs clear from both the block drain plug and the rad drain cock.
That should shift as much as possible of any loose “mud” so, when you then add it, the flushing agent will have the best chance of being effective.
B……
Bernie