To further complicate, i had tried mtime as a differentiator but:

• cannot rely on the mtime from the remote server and the files actually arriving in a sequence that corresponds to the mtime.
• I cannot either rely on the comm's link to the remote server being up
• cannot trust the files to be in sequence, but they will be in order, the gap will be unspecific.

But thanks for the above, to all, I have something now that 'will do'