Hi Hugh,
Unfortunately there isn't much you can do with this.
If it is only slight distortion then it is possible that an audio editor, such as Adobe Audition, might work as it has a filter that detects the level that the distortion ("clipping") occurs and flattens the peak above it. It could make the sound smoother but it could also change the whole dynamic of the recording.
If the distortion is really bad then I'm afraid you're stuck with it. There's no way of correcting the original recording in this case.