The ka and fiesta are fitted with a 'Heater control valve' situated near the wiper motor on the bulkhead, 99% of the time this is the cause of the above problem.
It will cost around �40 to replace from ford, �25 on ebay or �10 from a breakers yard.
It is very simple to replace (you only need pliers) - just a case of unclipping four hoses from the unit and refitting to the new one. oh and an electrical supply plug.
Just ensure that you state exact make model and year of your vehicle as there are two types of HCV available which look identical but differ inside, if the wrong type is fitted, it will cause your car to overheat.