Okay, I've seen everyone else's problems when they make a new worldspace and mine is along the same lines. After I've used the heightmap editor, the cells that I've edited dissapear and go to height zero.
I've been to all the tutorials and have followed all the instructions on how to fix this but IT WILL NOT WORK. When I go to flatten the land, it simply doesn't flatten (or even move at all) It sits there, a gaping rip in my
worldspace that I CAN NOT FIX. I've even tried to change the height differences and yet it still remains at zero height.
Can someone please help? I'm starting to get very annoyed with the heightmap editor and as my world is a large one, it wouldn't do for me to manually change the landscape.
Thank you.

The problem is likely related to having too much distance between points in your heightmap. Having distances greater than 4000 units between landscape points (not cells, the individual verticies of the landscape mesh) will cause that whole cell to become bugged. If you have any steep portions of land, you may also notice that you can no longer raise or lower points near these steep sections. This should be a hint that you are reaching the allowable distance between points that the game will accept. Once past this point, the only thing you can do is level the terrain to a lower height, or use the smooth tool.

Even if none of your landscape has steep faces, there are tears that always happen at the corners of each quad which MUST be fixed before you try saving. You will be notified of this error upon saving unless you, for whatever reason, decided to click the "yes to all" button at some point. As a general rule of thumb, you should NEVER EVER EVER click the "yes to all" when you plan to save the mod you are working on. Nearly all of the errors that pop up in the CS (except for the couple about shaders or alphas) are important and should be acknowledged.

Thank you so much, I believe that's fixed my problem. You've saved me hours of agony.

So, I've done as you've said Vagrant0 and it worked for a little while. When I went back into my heightmap editor and tried to change something though,
I got the same error messages and the cells I had just fixed were broken again! Once I fix these can I not use the heightmap editor again?

Thank you

You can use the heightmap editor again, and keep using it until you've placed objects in any of the cells you're editing (can use it after, but this can cause problems due to objects floating or being burried), but you would need to go through and fix the errors after each time you save the map.

Thank you, this has also saved me hours of problems