it's your heater control valve,a common problem on fiesta and ka. i'm an apprentice mechanic at a ford dealership and we do about 3 a week!
it sits up under the little panel below your wipers and a new one is about 30 quid so not too bad if you can fit it yourself. it the control valve goes wrong it usually goes to default and blows cold air all the time even though it would appear (by the heat in both hoses) that hot water is going to your matrix,it isn't! DON'T get a ford dealership to fit it or you'll end up paying about �150 repair bill for parts/labour! tip,clamp both hoses with small g clamps before you remove them and then you won't have to drain the system and refill it again,simply change the heater control valve! good luck! gonzo's lad.