Hot water combi boiler water not hot now making noise

[manaka]

I had a new central heating system put in 2002 a potterton pro forma 28, The boiler worked fine then I got a new bathroom put in 2007 one year later 2008 the water was not getting hot and the rads were when water run, so found out had diverter valve problem. You could not hear the water being run at this time, anyway I got a new boiler put in novemebr 2010 and the water still not getting hot in bathroom. Installer says the bathroom pipes must be crossed somewhere and came yesterday we took the ceiling down to look at pipes they are all okay no crossover he is now saying it was from when the first boiler installed but i had no hot water issues till 2008, the new combi he condensing boiler is making a lot of noise when it is run but more impotantly the noise from the cold water being run goes right through the boiler and you can feel it feeding through the pipe to boiler even water being flushed to toilet or running taps in the sinks. The installer says that there is a crossover somewhere and nothing to do with him, he didnt do any pipework as he replaced the boiler with a main boiler that is made by potterton the boilers look the same. im going to check the installation leaflet for both boilers as I have the manuals. I just dont understand why the cold water is going through the boiler to flush chain and use cold main. sorry for the long post its driving me mad oh by the way the installer reduced mains pressure and water is a bit hotter. he also asked me to run hot tap turned something off at boiler and showed me the cold water still coming out of hot tap which i dont fully understand but he says its the cold water being crossed over with the hot somewhere in the pipework and that its mixing with the hot to make it warm. I just think i would have had this problem all along