To make life even more confusing I'm getting different results with different versions of SQL Plus on the same PC! When running a 8.1.7 sSQL Plus I get 4 and when running a 8.0.6 from a different ORACLE_HOME I get 5. THe server version is 8.1.6.3.
I was thinking it could be an NLS problem i.e. some regions start the week on a Sunday and others on a Monday but I'm not aware of any NLS parameters in the sqlnet.ora file.
Any help would be much appreciated!