Since posting the above, spot check shows a number of assignments that were not converted have been, the number of records with the status of -1 has been reduced to 1153, and the number with the status 2 is now 2776. There are 349 rows where the converter field is NULL. (There are strangely fewer records in mdl_file_conversion (3929 - down from 4393?)).
So I presume MDL-56122 is indeed working - somehow, slowly.
In addition: Since upgrading to 3.5.5 and there is now a helpful message - that flashes briefly - where file conversion has not occurred: