175 DCI Temp sitting normal, heaters blowing cold / luke warm / hot.. ideas?

Hi all

First problem i have had with the car, only noticed when trying to demist the windows and want to get it sorted before what little summer we have is gone!

Any ideas what it might be so i have some clue before i hand it to the warranty company?

Temp guage sits normal on the car, but the heaters (digital climate) are blowing cold, sometimes lukewarm, and sometimes red hot as normal with no apparent pattern!

Any help is much appreciated.

Matt