I have now been able to open the files on my Mac. The original files were created using ‘OpenOffice’ and then encrypted using AxCrypt on a Windows computer. Some years ago I transferred the files onto my Mac without any problems still using the AxCrypt encryption. After downloading OSX Sierra the desktop copies of the files could not be opened but opening the OpenOffice app all the files were listed and I could open them using my AxCrypt password as normal.
I can only assume that some of the AxCrypt system is embedded into my OpenOffice since I believe that AxCrypt for Mac does not exist yet and is not shown as a application on my computer.
I do not fully understand why i can now open the files on my Mac but just very relieved that I can. To be on the safe side I am keeping copies on on a Windows computer.
Thanks again for your help.