You can hire the powerflush machines and do it yourself, I spent a full day on mine a few years ago flushing it backwards and forwards over and over and turning all rads off except one then emptying and refilling and doing a different one, the flush machines go in reverse and you can empty and refill the entire system in 20 minutes, flush, reverse flush, empty, refill and spend hours on it. In 20 years of CH, there'll be 10kg of thick black sludge in the pipes and rads that needs removing
