Microsoft Excel Excess Formatting Cleaner Add-in

It brought the file size down from about 3.5 to 2.5 mb, which was the best result I have managed to achieve thus far.

Fow whatever reason Microsoft didn't feel the need to say how to run the file cleaner, just install it. I found that somewhere,  but I thought I would mention it here. Once you install it, which is explained on the download site, just open a workbook and then open the XlsFormatcleaner.xla file. Then

click File -> Clear Excess Formats (which should appear just below Close). Then save. You may need to unprotect all your worksheets first!