Wow Jim, thanks for the prompt response. The surface coming up in FSX matches the "Grid" at ground level in GMax. Here's a screengrab. In Gmax it can't be selected for deletion, and turning it off before export doesn't make any difference. Don't know where the purple texture is coming from....
I'll go the "Export Selected" route and report back...
Worked like a charm thanks! Must have created something without realising it in the maze of frustration trying to make "friends" with Gmax on a first project. Now for those black textures...