Limescale build up is a mixture of calcium and magnesium compounds found inside your water. These are naturally occurring in some parts of the country.
The white chalky deposits left by this "hard water" can reduce the life of ceramic cartridge valves. They can mark the tap and sink over a period of time.
Water with a limescale content when dry, will have a white chalky appearance and will generally gather around the diffuser, the base of the spout and the body of the tap. it will also gather internally inside the tap and the ceramic cartridge.
A simple remedy to remove limescale marks and deposits is to dilute white vinegar with water. Then, using a soft cloth, soak the effected areas, then rub them with the cloth.
The longer the limescale has been allowed to collect, the more time this will take to remove. An old toothbrush may be helpful for tight areas.
Do not scrape the limescale off with a sharp object, as this will scratch or damage the finish on the tap.
The diagram below illustrates affected areas of England and Wales: