On most of the thermostats there are two sensors, ones goes to the ECU, the other goes to the dash. Unfortunatley when a thermostat is replaced, they normally only come with one sensor and the other is taken out of the old one thermostat and inserted into the new one(normally the dash one).
It could just be that you need a new sensor on the thermostat for the dash, but the car is actually running at the correct temp, but you just don't see it as being correct. It's worth hooking the car up to Alfa Diag, Unidiag or similar to check the actual ECU running temp. I've just brought a new sensor for the dash from the thermostat, £10.