It seems to be the norm for CH maintenance people to tell you your system needs "power flushing" at exhorbitant cost to you.
This involves connecting equipment to your heating system and pumping chemicals round the system and filtering the sludge out in the process.
A MUCH cheaper thing you can do yourself is to drain the system down, refill adding a flushing agent, bleed the system and run it normally for a few days. You can drain and flush the system until you're happy with. When it seems to be working OK then drain it again and refill adding an inhibitor.
This won't cost anywhere near £600.00
http://www.wickes.co....221226/?source=123_74
http://www.wickes.co....221227/?source=123_74
If you can't do it yourself see if a friend or family member can help you.