It seems that the bumps aren't to do with anything I did in Wilbur, because as soon as I save the map as an mdr file, if I import that straight back into FTPro, they appear. To get rid of them, I think I'll have to blur them out like suggested, but the 3x0.5 wasn't enough.

Is there any other way to get rid of them? And should I try and do it before or after all the erosion is applied?