You have to drain down a towel rad or an ordinary rad and pour a bottle of Sentinel X400 or equivalent in and then leave in for a month during winter when the heating is on a lot. Then drain, fit a Magnaclean or equivalent, and refill the system, do a mains flush, turning each rad on one at a time. Does require some knowledge / experience, but not a highly technical operation. An experienced DIY mate might know how, rather than employing a £ 100 per hour plumber to walk around your house turning rads on and off.