Actually, the water that runs into the 'tank' (as it's called here in the U.S.) is always colder than the surrounding air. Same thing occurs in airconditioned bathrooms on warm summer days. The difference in temperatures causes the condensation, which is aggravated if there's warm, moist air from a bath or shower.
Consider this... remove all of the water from the tank (excuse me, 'cistern') and dry it thoroughly. Obtain a small sheet of Rigid Foam insulation from your DIY store (looks like this:
http://www.owenscorning.com/around/insulation/ products/foamular.asp , DIY stores sell small sheets and often 'made to fit' kits just for this application such as this:
http://plumbing.hardwarestore.com/51-291-misc- tank-interior/toilet-tank-liner-kit-with-adhes ive-671112.aspx) Using any good waterproof glue (the ready made kits have adhesive already applied) cut and glue the insulation on the inside of the tank, lining it completely. This will quickly and effectively solve your problem without a tray of water standing around becoming smelly...
There is a special 'mixer valve' that can be purchased but it's installation is usually done by a professional. Seen here:
http://www.thisoldhouse.com/toh/article/0,,218 961,00.html ...Best of luck!