Excel 2003 - formula returns error when i copy a worksheet

Hi! Well this has been a humdinger! It seems completely illogical.

I have a worksheet that calculates various dates.

You enter the date the student pack was sent out (via the post). This cell is then read by another which calculates a start date 5 working days later, taking into account a list of holidays. That cell's (I7) formula looks like this:

=(WORKDAY(I7,5,AF3:AF133))

A timetable containing deadlines is then created using similar formulas.

There are multiple worksheets containing these formula and they all work fine.

However, when I attempt to either insert a new worksheet and copy and paste the contents of an existing one into it, or, if I copy an existing worksheet, the cells that read I7 to calculate deadlines etc show the following error if you change the date the student pack was sent out:

#NAME?

All the formulas are the same in the copied worksheet - so how is this error occuring?

Please note that this file was originally put together using OpenOffice.

I hope someone can help me with this!

Many thanks

Stig69