Hi Simon,
There are lots of ways to do this, and choosing the best way might depend on your needs. Here's what I would consider the easiest approach from the requirements that you mentioned:
http://share.smartbuilder.com/public/support/multiple-question-banks.mp4It uses our templates, and requires a little bit of custom logic - but because there's only one question from each bank, it's pretty easy. If your requirements change, it might not be the best approach. We include a little section about how to set up randomization with pulling more than one question from each bank, so if anyone wants the page that has that logic, please email support@smartbuilder.com and we'll send that over to you.
In the future we may update our templates to make this easier to set up. If we do that, we'll update this post.
Hope that helps!