OK this is mainly for delphi developers however it may have some relevance to other languages.

Firstly in delphi using activex you cannot have optional parameters,
we found that you must use EmptyParam if you wish to pass no data, we were using NULL or ''.

Ok the main reason why the above code wasnt working was that we did not need a DivideByField so used an empty string in place for this param.
this is why we were getting the error because (we think) that it was looking for the field name '' which is why it threw the exception.
we then found out about the EmptyParam thing and used that instead and this cured our problem.

Hope this is of use to others!.