Naturally soft rain water flows underground and picks up the minerals Calcium and Magnesium. These are the minerals that make water hard.
Guidance issued under the Building Regulations indicates that where the water hardness exceeds 200ppm, provision should be made to treat the feed of water to water heaters and the hot water circuit of combination boilers to reduce the rate of accumulation of lime scale and the consequent reduction in energy efficiency.
Whilst this guidance only applies to new houses or the fitting of a new boiler, it makes sense to do something before your existing system scales up.
For an idea of how hard the water in your area is, click here.