"Wrongly titled here, since Fernox F5 is the "heavy duty" version. F3 is the gentle cleaner. I've used both. It is neutral, as in non-acidic, making it good if you don't want to expose any hidden leaks in the system. The F3 has cleared most of the magnetite (black rust) that was clogging my heating, without the use of a power-flushing machine.
It's hard to give a definitive review, as my situation is unusual: My heating is a 51 year old overhead gravity system with iron pipes, not cleaned for decades, and I'm doing repeat cleans with F3 and F5. Curiously, the heating works 100% while the F3 is in the system (not so with F5), but it has to be removed/flushed after 7 days max. But this cleaner appears to do its job. Good value here at Screwfix."