Polo 1.4 CL Petrol blows COLD air from vents and overheats when idle?

VW Polo 1.4 CL petrol, automatic car has just had the thermostat replaced and is now blowing COLD air and NOT Hot air from the internal vents, when air blower switched on. Cannot see any blockages, as pipes are off. The car fan does come on okay, and car only slightly overheats still on when in idle state after a while the needle goes quite high.
But whilst driving, it does NOT overheat at all as temp needle stays constant at 70C in middle - but still only givesd and blows COLD air from the internal switch, to keep the window clear or myself in this cold weather.
Can anyone suggest or help ?