Hello Barry and welcome to Seven Forums. My usual disclaimer: I'm not an expert in anything.
I believe that once a system image is overwritten you can't recover the original data. Microsoft says that once a system image is made, "...you can't choose individual items to restore, and all of your current programs, system settings, and files are replaced with the contents of the system image." What is a system image?
I believe that trying to recover the old system image would be trying to selectively pick and choose individual items. I could be wrong on this and if so others will probably jump in to correct me.
Could you still use this new system image as your "master"? It should be called WindowsImageBackup
on the external hard drive. You could rename it to something like WIBMaster011612
so it won't get overwritten again. The next time you create a new system image it will then be called WindowsImageBackup
and your external drive will show two files ... WIBMaster
If you ever need to restore to the "Master" just name it back to WindowsImageBackup and select it from your choices.