Cannot read Excel 2007 despite installing Compatitibility Pack

We were preparing to send to clients a large Excel 2007 worksheet with much
formatting, and many inter-spreadsheet links. We then discovered this
spreadsheet cannot be opened by users with Excel 2003, even if they have
installed the Microsoft Compatibility Pack.

This problem occurs regardless of whether the Excel 2007 file is saved as
xlsx (i.e., in Excel 2007 format), or is converted to xls (Excel 2003
format).

When attempting to open the Excel 2003 file, the following error message is
displayed:

“Too many different cell formats�
“Errors were detected in “GPG – Big Table.xls,� but Microsoft Office Excel
was able to open the file by making the repairs listed below. Save the file
to make these repairs permanent.�
“Damage to the file was so extensive that repairs were not possible. Excel
attempted to recover your formulas and values, but some data may have been
lost or corrupted.�

In reality, all the formatting has been lost, as have the inter-spreadsheet
links, so that the Excel worksheet is unusable.

When attempting to open the Excel 2007 file, the system prompts us to save
it as an Excel 2003 file, and the above is repeated:

What can we do? We cannot send to clients an Excel 2007 spreadsheet if it
cannot be used by clients who only have Excel 2003. Starting afresh by
inputting the data into Excel 2003 will take a long time. What is the
solution?