Add Sentinel X400 for a week, switch off heating, open all rad valves fully, remove pump, cap off feed and vent with push fits caps, then useing two hose pipes connect to pump connections, one connected to the mains and one to the drain, shut all off rads but one, carefully turn on mains and leave to run until clear, turn on next rad and then turn off rad you have just flushed, do this till all rads flushed, reverse connections and do the same again, rejoin cold feed with push fit coupling, uncap vent, only X400 going into eco system and heating thoroughly flushed leaving clean water, add X100 inhibitor and job done, should take a morning, two 22mmx 15mm conex reducers, two hose pipes, two jubilee clips, one pushfit coupling, two pushfit caps(re-useable), nowhere near £400.00 and no nasty chemicals bugggering up the environment which the ODPM "loves" so much and were all busting our bollokks to save..