Consecutive Invoice Numbers, Restarting Each Month

Hi there,
Some scene setting to my problem. I have a corporate system numbering invoices consecutively globally. For a subsidiary in Poland this presents a problem - all invoices for the subsidiary must be consecutive. ie. where the globally issued invoices for Poland might be 1,4,5,7 these need to be renumbered before issue in Poland.

It gets more complicated as the Polish Office standard is to invoice in the format "PL/Year/Month/Number" with the Number restarting from 1 for each month.

My database is very simple.
Table 1 = Link table to Oracle corporate data
Table 2 = Local table of invoices already produced (Global invoice number and date recorded)

I have a filter to find all new invoices (unmatched items), a report to print them and an Append query to populate the new items to the Local table.

My challenge: (1) to recreate the Polish numbering scheme and (2) have it consistently when existing data goes up to mid month and a new append operation has to follow the invoice number sequence.

Any ideas? I guess Autonumber is no good and I have little array experience.

Thanks