Mk 1 1.2HTP Fabia Estate - Cold Air in Heater Only.

Hi Folks. My first post and Im stuck big time with the Heater fan in the Fabia 1.2 Blueline of my Wifes Car. Air is cold at all times. I have cleaned out pollen filter and checked flap on heater dial which appears to be working normally. No overheating at all but a little coolant expansion when I take the cap off the coolant bottle. Car has no aircon system just normal heater. Any help to rescue my sanity would be much appreciated.