if you read page 2 of this link
and read what rossn70 has typed it sounds very similar to your problem and he fixed it by replacing the control valve. if you look on page 1 of the above link ive put a picture where the control valve is on the fiesta may be in a similar place on the KA not sure as ive never owned a KA.
if i remember right i think the valve new from ford was about £30
but if you read the link above you could test this by using two bits of copper pipe and byepassing it just to test, the heater will only come out hot regardless of where you set your temp controls.