Thank you that's very helpful. My local library uses Borrowbox, would have to check if they use Overdrive (that makes me sound as though I knwo anything about it, I don't!). So from your comment, you can only use Overdrive or Borrowbox from libraries where your normal library card would be valid?
Yes, you have to use your library card to log into your BorrowBox / Overdrive account (sign up via your library website). I assume that each library pays for access to certain books or something. My library recently moved over to BorrowBox and it's not as slick as Overdrive wrt to the Kobo. You have to borrow the book on your computer and then transfer to your Kobo using Adobe Digital Editions (free download). The first time is a bit faffy, but it's ok really.
For me the faff is worth it for the sake of 10 free books every 3 weeks!
This only works on the Kobo or any other open source e reader. Kindles are restricted to Amazon purchases only I believe.