My apologies. I'm sure there are other threads here regarding heater issues. Problem is, I don't know where to start. My engine doesn't overheat. The only problem I have is the lack of hot air from the heater. It stopped working about 6 months ago. For a while it would come on hot, then slowly change to cold after a while. Then I would have to turn it off for a few minutes and back on again. Then it would be hot again, for another while. But after a month or so like that it stopped working altogether, and now I can't get any hot air at all.
The first thing to check is whether there's sufficient coolant in the radiator. (If there's not, the heat exchanger won't be able to function properly). Otherwise, the heat exchanger has probably leaked. Fortunately, a new heat exchanger is one of the cheaper parts for a Fiesta!
I have just found the same problem with a car i just bought, under the bonnet there is a cable that the switch operates, (a bit like a brake or gear cable on a bicycle) this was stiff and only needed lubricating to repair the problem, hopefully it could be the same on your car.
My girlfirend's fiesta has just had the same problem, however changing the thermostatic valve fixed it. The valve is next to the windscreen wiper motor, and costs about �40.
According top the threads here, it's a fairly common problem with cars of this age.