Many years ago I had a similar problem on a Ford Escort. There was an air-lock preventing hot coolant from the engine block from getting to the heater radiator; bled the air out and behaved perfectly. Look for bleed-valve high up on pipework near heater box under bonnet.
There isn't a bleed on the upper pipes on a Fiesta. They are easy to bleed.
I suspect your thermostat is failed & stuck open a bit. Get a thermo and seal and replace it.