go on ebay or google type in powerflush companys are doing the for £250 to £350. and if you flushed the system it will clear the heat exchanger dont belive them they try it on i am a plumber in the midlands and i hear this powerflush all the time when i see customers of mine. it will block the dhw before the main heat ex most likely a good powerflush will clear the cause.
kash
September 2010