The above referenced preferred ionic monomer and other suitable ionic monomers may optionally be used in combination with a polymerisable sulphonate or a salt, e.g. an alkali metal salt, such as a sodium, potassium or lithium salt of acrylic (3-sulphopropyl) ester, commonly known as SPA. SPA (e.g. as potassium salt) is available commonly in the form of a pure solid from Raschig.