Power flush doesn't always clean the main heat exchanger, the way it's made if completely blocked it's gone, it has to be replaced with a new one usually cost £500-£700. a lot of people tend to change the boiler at this stage.. ..still power flush the system so that the new boiler doesn't get blocked. Power flush technically doesn't clean the secondary heat exchanger, it has to be removed and descaled separately or changed for a new one. protect it with a water softener or lime scale inhibitor...inline or electronic. Power flush has to be done at least every 5 years before any of the heat exchangers get blocked. also install a magnetic filter to keep the heating system clean the longest possible as corrosion inside the radiators will never stops taking place. add corrosion inhibitor once a year ..fernox f1 or kamco system safe Dm . ..don't go for cheap brands from ebay. protect your boiler rather then with BG or 3rd party plumber.