It will be the valve on the bulkhead, under the bonnet right at the back slightly to the left of centre there is two hoses going into it. I think the valve is only available direct from ford, about �30. dead easy to change.
What twazerk said.
There are two heater hoses going in from the engine side and two on the other held on by those spring clips. There's also a plug which goes on the solenoid that stops/starts the flow of coolant to the heater matrix. It's a very common fault on Fiestas.