Adding sequential number to a report ?

Hi,
I'm trying to create a sequential no which is put onto each report at the
top grouping level.
The report is a set of invoices, and the top grouping level is basically the
company name, with a group containing info on the each items invoiced,
price,tax etc below that.
On the top grouping level, I want to put a number which starts at one for
the first invoice, and increments for each time the report creates a new
company name section. When I run the report next time, I want it to start
from the last number used + 1.
So
Company A Invoice No: 000004
xxxxx
xxxxx
xxxxx
new page
Company B Invoice No: 000004
xxxxx
xxxxx
xxxxx
etc
The report is working fine, creating all the invoices correctly but I'm
stuck on how to do this number.

BTW The number doesn't have to be sequential, but must be unique. I don't
need a record of the number for each invoice issued, just to know what the
last number used is, so I can start the new report on the next number.
Any ideas

Tony