I have been using the G.E.C.K. to edit Fallout 3 but suddenly it has started coming up with an error message whenever I attempt to save and is saving my files as .tes instead of .esp so I cannot edit or play my newly created level, what can I do to solve this?

Never mind, I solved it.

This occurs if you have copied the original .esp, delete the copy and it should function as normal again