The Magnaclean is extremely good at capturing black sludge (iron oxide) that comes from corroding steel radiators, usually. You can add a chemical such as Sentinel X400 which loosens old sludge so the Magnaclean can capture it. You ought to be opening the Magnaclean yourself and scraping the sludge out. It is not much more difficult than opening a stiff pickle jar, and you will be amazed and delighted to see how much it catches in a week. Eventually (unless you still have a defect causing more corrosion to occur) it will stop finding any and will stay fairly clean for months on end.
Adey's Magnaclean was the first modern, mass-market product to do all that, and for several years it cornered the market. Unfortunately however the build quality is not first class and they tend to leak in time. The canister and lid are made of hard plastic, with a very powerful magnet sheathed in stainless steel inside. I understand BG stopped supplying them because they had so much trouble getting call-backs to leaks. I have used Magnacleans but in my own house I have a Spirovent, which is a very solid brass device that captures particles. However it is not as dramatic and fast in use as a Magnaclean. My system is very clean though so there isn't any black sludge to capture.
If you have a very dirty system I still think it is worth fitting a Magnaclean, even though you may have to replace it in a few years. By that time your system should be clean and you could have gate valves fitted to the pipes to make it easy to replace if necessary.
Before a new boiler is fitted to an old system, the old system is supposed to be thoroughly cleaned so all the old dirt is removed, and an inhibitor added to prevent any new corrosion occurring. Damage or clogging to a boiler caused by dirt and sludge is not covered by the manufacturer's guarantee; they sold you a clean boiler, and it isn't their dirt.