Thanks so much for getting back to me!
The file name convention at the moment is [Firstname][Secondname](underscore)[Export](underscore)[a four digit umber starting at 0001 with leading zeros][.pst]
There'll be several of these files for each user, so the number will be incremented.
As long as the new filename starts with [firstname].[secondname]@domain.com. then the number can be more or less anything, it won't matter about the leading zeros. (I think!)
Other (made up) examples are: