The series in question was Bleach 2004 (
https://thetvdb.com/series/bleach#seasons).
They are labeled with their absolute episode number so S01E01 is labeled 001, S01E02 is 002, S02E01 is labelled 21 etc.
I'm using TVDB.
I ended up doing it using a different program (rename my tv series), but if you can let me know how to do it in future it would be great.