I recently had the same problem. My turned out to be something called the heater Valve (between fire wall under bonnet, and widscreen toward passenger side). Picked the part up from ford for £30 and fixed myself. Was quite tricky and you'll need to be determined if you take it on yourself. Heater Valve "going" in fiestas is a common problem (so i'm led to believe from other forums).
Ged
October 2008