Hi jbb,
I have not yet received my certificates, its only been 1 week, but I hope it will arrive soon.
I assume you can apply for it before the case officer asks for it, you would need to state in your application that it is for immigration but before its been asked for.
I contacted the Dutch embassy in London for advice before I submitted mine. You can get their number and email address from their website, they are very helpful.
Good luck