It appears that cells containing formulae that reference other cells are 
sometimes stored as zero when saved in Microsoft Excel 97/2000/XP (.xls) 
format.  For example, if I open the affected file in Microsoft Excel Viewer the 
cells display zero instead of the correct result.  I'm afraid I've not worked 
out how to reproduce this but it has happened to me on a number of occasions.  
If I make some kind of trivial change (again I haven't worked out what triggers 
it) and save it again, it fixes itself.

If it's of help - when searching for this in the issue tracker I came across 
issue 9895 which appears to be the same problem but was apparently fixed 
several years ago...?

I will attach a file called cells_links_broken.xls that is currently exhibiting 
this problem.  An example affected cell is _Optics.A42.  It does seem that it 
happens to cells that are merged together, but this could be coincidence.

Thanks a lot,
Emma