Generating a unique reference number

Hi

I would like excel to create a unique reference number or I can get it to look for a particular reference number from another document that I would like it to use however when I save the document I would like it to then when I open up the template document again it would have increased the number

ie first reference LTB/0001

when I save the document and then I open the template document it automatically has LTB/0002 etc.

can anybody point me in the right direction

thanks