I put a Magnaclean on my mum's CH - when I swapped the boiler I spent a long time flushing the system through, and I then followed the Keston advice to add a Y-strainer. Every few weeks this would clog up with something the consistency of grease. In the end I took it out and replaced it with a Magnaclean which doesn't obstruct the flow as it collects stuff.
Yes, I know that this shouldn't be necessary in a perfectly sealed system, but in this case some of the under-floor pipework dates back to the 1960s, compression jointed. There's no major leak - system needs a top-up every few months. Ideally the pipework should be all replaced and the system zoned, but with laminate and vinyl floors the disruption would be horrendous.