Greetings Noel, and best wishes for the New Year.
You are right, Norfolk Water is some of the worst in the country for boiler feed. It has a very high Total Dissolved Solids (TDS), and these dissolved materials are left behind in our boilers when we boil off the water. typically Norfolk has TDS levels of 250 to 350 ppm, whereas the Power stations aim for about 0.2 ppm.
Rainwater is about the best thing to use, water softeners are not much use, since they could introduce chloride ions into copper boilers, nota good idea.
Low concentration hot citric acid solution, is best for boiler cleaning, together with copious water washing afterwards.
ME published a very interesting article, "Water Water, every where" a while ago, dealing with this item.
Regards
Brian B