I don't know anything about the eSafe product, but doing a cursory look I see that its web content protection product. Is it possible its now blocking http downloads from your Casper server, assuming you have that enabled on your distribution point?

What are the error logs in Casper indicating as the problem? Also, have you tried overriding the default settings in a Casper Remote push or a policy to force AFP/SMB over HTTP as the download mechanism?

I am working on this issue with Haydos.

The only message given by the log is "The Install failed (The installer encountered an error that caused the installation to fail. Contact the manufacturer for assistance.)"

Yes, we force AFP/SMB over HTTP.

Further to this:
The specific product in use is "e-safe Prism" client software, and not the filtering product from Alladin Systems.

This e-safety software product is installed on the device to protect users from e-safety risks and has functionality to control certain types of files, applications and web content from being accessed. As .pkg files are prevented from downloading/installing onto a machine after it is installed (with no obvious blocking features being enabled to do this) have any other Casper users found any other security applications installed on the Mac O/S appearing to inhibit or prevent .pkg packages operating successfully but allow .dmg to work happily in the same environment?

Feedback from another site using this e-safe Prism software with an earlier version of Casper indicates that they do not have this issue so am wondering if my problem relates to a specific change to Casper, O/S update or file permission issue as suggested with other Casper deployment related issues for other applications.