Ford Mondeo heater problems

I have a lack of hot air blowing into the car. I find that the temp gauge always sits at the lowest position, while I am moving. When I am sat in traffic the temp gauge does move up and reaches the centre position on the gauge. Then as soon as I start moving it rapidly drops back down. This means that I can't get hot air blowing into the car, and this time of year it is blooming cold.
Can this be something I can fix myself?

Thanks