hi it will be the heat exchanger they have small fins that the water passes through as it heats it this is seperate to the main heat exchanger that does the ch these fins get cloged with limescale the flush will cost you £500-£1000 i would buy the ehat exchanger yourself from web around £70-£80 and a limescale remover again bought cheap then pay an engineer to fit them. I was getting charged around £175 for the exchanger and £50 to fit it and i did this 3 times in one year i then got sick of this and ended up buying one for £60 and fitting it myself so good luck and let me know if you need any more help.
kam, June 2009