Cheers Fredrik.
I tried the translation program and can make no sense of the code it's
producing.
However I did make me think about manually translating VBA to C++, since the
classes are similiar and there is some help for them in VBA.
I have just realise that I can use covOptional for optional variants and
have been able use CWorkbooks::Open() method to open my file.
Thanks for pointing me in the right direction.

Hamish