No, the include_blank is not the problem I think.
date_select automatically selects the value given in
@enquiry.date_of_birth...
The problem must be somewhere else. Are you sure
@enquiry is not reset? A mistake I used to make is to
redirect from the create to the new action instead
of directly rendering the new template from the create action
when rerendering the form.
anyway, this is just a guess...
I need to see how exactly you are rendering the form.