Corrosion is generally a process of oxidisation of metals which in a central heating or cooling system is promoted by the following:
Incorrect, unsuitable or poorly applied or maintained water treatment products.
Ingress of air, for example, at the mechanical or poorly soldered joints.
Some plastic pipe allows the ingress of oxygen into the system water, unfortunately some early on plastic pipework is now deemed unsuitable for the installation of certain heating systems, this problem was overcome by the addition of an oxygen barrier being fitted, in modern pipework a vapour barrier layer is fitted.
In some cases you may not be able to identify the substandard pipework but where it is identified it is recommend to be replaced.
Poor system design and installation, examples; oxygenation of the circulating water by replenishment, system pumping over or sucking down on the open vent, incorrect sizing of the cold water feed pipe, inadequate sized or incorrectly installed feed & expansion cistern (F&E Tank), incorrect configuration of feed & expansion tank.
The presence of installation materials in the system water, examples, flux residue jointing compound, this is a residue cleaner foreign matter left over from inadequate cleaning or flushing. Oil greases left over from the manufacturing process of system components.
Certain characteristic of the system supply wate, for example, some naturally soft waters with low alkalinity and low PH are generally more corrosive to system metals. Some water may contain high levels of aggressive irons, example, higher chloride levels can promote pitting of metals.
The presence of bacteria, especially in poorly maintained open vented systems does have a serious cause for concern due to the build-up of legionella bacteria in open vented (F&E tank systems) which are now obsolete and should b replaced with a sealed system confoguation which will significantly lower the risk of exposure to legionella bacteria.
The comments is section 5 are highly unlikely within the South of England as the water is deemed to be hard but it can be a problem if people have had water softeners installed.