I am amending the expense claim my company uses and I want to be able to
identify each claim by a unique number.

I have played around with using time and date functions to create a
reference number, but I can't really do what I want (It was a pretty bodge
idea to begin with, and I don't seem to be able to get the time reference to
update often enough to be really valid if an employee has the same expense
claim open constantly. Plus of course if they reopen the document later...)

A formula or piece of code that creates a reference one alpha/num more than
the last time the document was opened would be perfect.

All responses gratefully received.

Cheers, P