The probability is the heater valve has failed. It is located under the bonnet in the centre near the windscreen.
If i were you i'd pop into a ford dealer & ask one of the technicians for advice... Don't go into the reception, just hang around the reception carpark & ask one of the boys in blue for advice. Most are fine.
If your advised to buy a valve then ask for trade discount, again most will oblige. Alun. Its a common fault on KA's
Alun
January 2008