The Ford Ka uses an electronically controlled heater valve which is located near the engine bulkhead. This is most likely to be the problem as I change loads of them. It could be the switch not sending a signal to the valve, but this is less likely, but you need to check the signal at the valve to check the switch first.
If it is the valve, as it usually is, it is quite easy to change, however one point. When the vehicle is topped up with water/anti-freeze the car needs taken for a short run to bleed the system. When you return make sure the heater gives out hot air, allow the engine to col and check the water level again.
Hope this helps
Steve Eastman, February 2005