Hi Neil. The symptoms you describe are indicative of a failed thermostat. As Charlie says above, start the engine from cold and wrap your hand round the top radiator hose. You shouldn't notice much heat developing in it for a while because the thermostat should be closed, could be up to 5 minutes if it's a cold day? Then, all of a sudden, the thermostat will open (most open around 90 degrees C). The hose should get very hot very suddenly, you should not notice a slow steady climb in temperature. This is because when the engine is cold the thermostat should be shut which stops the water being able to circulate through the radiator - it will still circulate through the heater though. So, if the thermostat has failed open - as they often do - (or it's internal rubber seal has failed) the water will circulate through the radiator right from the moment you start the engine, this results in the engine trying to heat up a much larger volume of coolant so the heater takes ages to warm up and, because there is no temperature control of the circulating coolant, it'll be unlikely to achieve a "proper" running temperature so the heater never gets really hot.
It's a common thing to happen, not just on our wee FIATs but on many makes of cars. The thermostat leads a hard life as once it has opened it is constantly shuttling around slightly opening wider and then slightly closing down ad infinitum so as to try to maintain a constant hot running temperature for the engine - very important with the fine control which modern electronics apply to the engine.
There are other things which just might be causing this, are you hearing "gurgling" noises from behind the dashboard? that could be an airlock. Have you checked the header tank for the radiator? is it up to the mark? Any obvious signs of leaks? However, from your symptoms it's most likely to be the thermostat. I've replaced the ones in our own Panda (2010 1.2) - which was struggling to reach the quarter mark, and my boy's Punto (2012 1.4) very similar petrol engines in both. The job was pretty easy although there are a few bits and pieces to move out of the way for access.
I made this post when I changed the thermostat on Becky (our Panda)
https://www.fiatforum.com/threads/what-did-you-do-with-your-panda-today.436409/page-64#post-4581167 She has the earlier type thermostat which is a bit cheaper. The newer engines have a housing which includes the take off stub for the heater and the water temperature sensor. If your's has the later type it's worth looking for a replacement which has a metal heater take off pipe. Many are plastic and people on here report that they tend to fail with age.
