Not sure if it is absolutely necessary but I used de-ironised water you can also use distilled water instead of tap water.
Oooooops , didnt update .....couple of weeks ago now , but , drained the system again and flushed out with garden hose , refilled with radflush and ran for 20 mins or so , drained system again and flushed through , most of the brown stuff , not really gunge as not thick enough , was at the bottom of the header tank so put the hose on the filler and flushed it out through the overflow , refilled with fresh anti freeze , that was about 700 miles ago , is a bit discoloured now but a lot better than It was , will do the same again in a few weeks , almost certainly caused by not changing often enough