Dear Linc Davis,
Thank you so much for the effort you went to in order to provide such a comprehensive solution. I really appreciate it. I checked Mail again this morning and, bizarrely, all the Rules have returned. A friend just sent me this link, it seems it's a known issue following upgrade to Mtn Lion.
Again, thanks for your help
OS X Mountain Lion: Mail rules may not appear after upgrading from Mac OS X v10.6
When a system is upgraded from Mac OS X v10.6 Snow Leopard to OS X v10.8 Mountain Lion, existing rules may not appear the first time Mail is opened.
Quit Mail and reopen it. The Mail rules should appear.