Rather then abandoning the import of a file when an unknown (i.e probably new) MPAN is encountered, and thereby leaving out even the data that's okay, alter Chellow so that the data will be imported, and an error logged somehow to report that the file was processed, but not perfectly.
Perhaps by adding a junk supply to hold the data for the suspect MPAN, and then carrying on with the rest of the import?
The existance of 'temporary' supplies could then be reported as a site snag to flag up that they need to be fixed properly.