You will always get some corrosion that sits in the bottom of the rads, it's just the nature of mixed metal systems and oxygenated water and that's ok. No matter how much inhibitor or filters are fitted it will always happen to a certain degree and then it should stop once the oxygen is used up and the inhibitor does its job. The trick is minimising any build up and ensuring it doesn't get so bad that it ends up in solution (black system water) and gets to the boiler.
The section around the feed and vent is always the worst for some reason, well known for blocking up
Many thanks for the positive update @wardr77 ... sorted
