Hopefully you've fixed this by now, but if not, it's not a simple job I'm afraid.  Punto heater matrixes are prone to corrosion if the anti freeze isn't changed religiously every 2 years(that goes for all Fiats and Alfa heaters as well)   Fixing it will usually involve removal of a fair bit of the dash unit, so unless you're VERY handy with the spanners, I'd save up a few quid and get it to your local garage.  

Tim