Renault Clio 1995 slowly overheating?

I have an old Renault Clio 1995 1.2 RN. Bought it a year ago and it has always had a problem with staying cool. (only 62,000 miles)

The temp gauge rises straight to halfway which is fairly normal, but it will only stay at halfway if I stay moving. As soon as I stop in traffic it will continue to rise. I haven't let it bring on the red warning light yet. I can usually keep it cool by using the cabin heater and fan on max and the windows open :)

A garage told me that the head gasket is going, but running this by a friend he asked "Is it loosing water?", well it's not loosing a drop, coolant expansion vessel level is the same as it was when I got the car.

I'm also told that the fan should come on if the temperature rises about halfway, but that the gauge shouldn't go above halfway unless something is wrong of you are driving it too hard. I haven't checked the fan, but I will.

I'm going to sell it anyway, I'm just determining how much of a lemon I'm passing on and whether to sell for parts/repair or as 'road worth'.