I have a problem with a the heater in my car. It sometimes blows hot air, and then at time completely freezing cold air.

Its a recent problem with a 2003 Skoda Octavia. The engine temp is fine there doesnt seem to be any flunctuctuations by during driving just the air issue.

All the radiator pipes are hot. The only issue being that the coolant level slightly reduces after a few days. I have topped it up completely at the moment, but I am not sure exactly what the problem is.

Climate or manual?

Manual

I know the fabia has cabin sensor problems. Might be worth checking that

The heat issue sorted itself out however the car 'drinks' coolant and I am loosing small amounts of it. Someone suggested that it may be a stuck thermostat.

Stuck thermostat won't cause coolent level to drop, its going somewhere, either burning it or dripping it somewhere,

Have a scout round car for signs of water marks, start round radiator area as thats most common thing on most cars

Also if the car is left to stand for a few days, how does it start then? Fine, or a bit more white smokey than normal?