Press the self clean button , hold it there, and gently move the iron to and fro, over the basin or bowel. CAUTION: Keep clear of the escaping water and steam. Water and steam will wash scale and dust through the soleplate, and into the basin or bowl. When the water tank is empty, release the self clean button .
Nowadays there are various services for doing such work .You can hire them.There are also leak detection services who can help you in finding the leakages,repairing pipes ,removing mold and cleaning the tank.So it is better to hire them and check your pipes while cleaning. http://www.allkarepropertydryingout.co.uk/luton/leak-detection-services-london.html