Thanks to everybody for clarifying this for me. It still doesn't solve
my problem of how records that should be NULL are actually empty
strings - this should not have happened and I still cannot see why it
should have. Maybe it is our application after all. But if it was the
application then I would have hoped to see the same behaviour on
Oracle, but I do not, all the equivalent fields on Oracle are NULL
rather than ''.

If I do find anything I will post the results back here.

Thanks to everybody for their time,

Paul