overheating problem

Firstly i would like to say hello to everyone and thanks for a great forum. Ok, i have a MGF Trophy 160 Y reg that tends to overheat rather quickly if sat in traffic. This however can be overcome if i leave the heater running hot all the time. The water pump seems to be running fine as i can see circulation at the water reservoire, also the fan comes on but seems to do very little as far as cooling goes. It first happened to me a few weeks ago when i noticed the engine getting hot and saw steam in my rear view mirror, i quickly shut the motor down and added coolant, left the heater on max and everything has been fine, until today, i forgot to turn the heater on and it did it again! Maybe a sticking thermostat? Any help would be geatly appreciated...

P.S no water in my oil.

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