Brick acid from B&Q. Put it in the header tank and then attach a solution hose with an open ended fitting and run it around the system and back into the header tank. Leave it for half an hour, start the machine up and run it around some more. Leave it another half hour and then flush it out with a continuous feed of fresh water until all of the acid is flushed from the system, this can take 10 - 15 mins.
Failing that use HM Clear Water Rinse in the header tank and do the same process but it will be less effective.
Simon