How accurate is the 'calories burned' figure on my GPS device? I use the Garmin Edge 200 to record the route, distance and speed of my rides. It also gives me a prediction of the amount of calories I have burned on any particular ride. 

The only data I have given it is my weight. It must use the speed and elevation GPS data to calculate a prediction but id like to know how accurate that can be if it doesn't take things like wind strength &amp; direction, quality of surface and tyre type into consideration. 

As an example, I went for a 50km ride today and it informed me I burnt 1200  calories. Is this in the correct order of magnitude?
By order of magnitude if you mean by a factor of 10 then most likely yes.

You have wind, rolling, and gradient resistance.   

It is going to be spot on for gradient resistance so if you have a hilly ride it will be more accurate.  

Clearly it does not account for wind.  

I expect it assumes a mid level road bike on decent pavement. 

At speeds over 10 mph rolling resistance is the smallest of the three.
So it is going to be off on what is typically the smallest factor.
See the graph in the link:
http://www.schwalbetires.com/tech_info/rolling_resistance 

On a hilly road ride with no wind it is probably with 10% overall.  

Riding flats drafting in a group it is going to be off by a lot.
Trail riding it is going to be off by a lot.