The water seems to be just dumped out almost in one go, but this only happens every few days. Thanks in advance

heater could be blocked or it is because u have no coolant in ur car.get the leak fixed and ask them to flush heater pipes as well

The fiesta, puma & ka all have a little valve at the back of the engine bay, which is controlled by the heater console is hot & cold knob. When the knob is turned to 'hot' the valve opens to allows water from the engine is coolant syetem into a little radiator in the dashboard, which warms up & air that is drawn in by the fan passes over this little radiator & warm air comes into the car.

These valves r really problematic & they often fail. The most common symptom is that the valve jams in position, whether that be open or closed - hot or cold. Then the heater can not be adjusted & it is either always hot or always cold.

These valves do do other wierd & wonderful things & it would not at all suprise me if it was leaking in some way & effecting the operation of the heater. Whether u get a new one or second hand one & fit it yourself or have it fitted or take it to a ford dealership, I'd definitely start with that. l believe it is called a heater exchange valve.

Good luck