A follow-up for those interested.
Re-uploading a fresh SOE image to Fog solved the issue: once I had done that it was fine. Still not 100% sure why I had the issue with restoring the image files I restored from backup, but maybe it could be related to the following…
I use the standard Fog backup script, which does a good job of getting all the important stuff. One thing it doesn’t backup by default though is the /tftpboot/ partition. Now, you don’t really need to back this up unless, like me, you have made changes to anything pxe-related.
In my case, I had applied the chainloading fix described here:
[url]http://fogproject.org/wiki/index.php?title=FIX:_Chainloading_alternate_SYSLINUX_to_force_boot_from_first_hard_drive[/url]
Of course, that was ages ago and I had forgotten about it…until all of our newer Dell machines (optiplex 790) started rebooting when they tried to load the pxe boot menu 😞 Once I reapplyed the fix it was fine though.
The end result is that I manually added the /tftpboot/ partition to my fog backup scrupt, just to make things easier next time I have to do a restore (hopefully not for a while!).