Onion, I don't think the catchment area is as big as you think. Half an hour up the road from MME and your nearer to NCL, half an hour down the road and your nearer to LBA. On routes served by all three airports i'm pretty sure there's not much difference in price, so thats not a factor. I think most will choose to fly from their nearest (by travelling time) airport, business type people will choose to fly by the frequency of the flight (there and back in the same day). The two reasons stated above shrink the catchment area of MME. There are a few exceptions, unavailability of seats at other airports and last minute deals but i think on the overall scale of things they are few and far between. So in my opinion the rough catchment area is Durham City to the North and prehaps Ripon/Thirsk to the South.