Why do you think there is a problem with the freshwater circuit?
I can understand the heater exchanger becoming blocked after a few years of use. The last one I had came apart and went into the dishwater whilst the wifey was out. Did quite a good job of cleaning it up. Turbo intercooler went the same way to get the oil deposits out.
I just put in a new waterheater. When I disconnected the engine heater hoses, a lot of calcium/scale came out. so I thought I should flush it all out. By the way Pete, I left Birkenhead in the 60's to come to California. Ex Merchant Navy.