I have searched (pretty hard) to find a valid reason why excel might
reset macro security back to high (from medium) whenever it gets the
urge!

I have somebody who can be working with an excel document quite
happily and then all of a sudden macros will stop running and they
will get the "Macros are disabled because the security level is set to
high....." dialog. Resetting the security back to medium, closing and
reopening stops this but obviously it's not ideal. I have been
informed that there are no group policies set up either.

Anybody have any ideas?

Thanks.