You may want to look at the way J.E. McGimpsey approached it:
http://mcgimpsey.com/excel/udfs/sequentialnums.html

Denni123 wrote:

I am trying to create invoices n Excel that will insert a unique reference
number each time thetemplate is opened.

I am not sure which function will work for this application, or whether I
need to create something in visual basic.

Has anyone got any suggetions?

Thanks

David

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Dave Peterson