Are you joking? You an agent?
How did we do this easily before ruddy Powerflush tried to fleece us all?
Drain down a couple of gallons by draining at a drain cock or radiator connection (catch the stuff well .. it may be black and WILL stain ANY carpet) You may have a sealed system so take into account refilling/pressure/fill loop, etc. Turn off water rising main to header tank if fitted (or hold ball valve up)
Add Fernox products as described here ...
http://www.fernox.com...entral+heating+system
Run the system hot for the day, and drain and refill completely adding inhibitor.
Easier than it sounds. Made easy by a decently positioned drain cock, to which you connect a hose, and the thought of saving £300 on Powerflush.