But I'm not having any success with either method.
Anyone got any tips?
"You can loan eligible Kindle books from the product detail page of a book you purchased on Amazon."
Does the book's product page state that Lending is Enabled?
You're posting on the UK forum but are referencing a Help page from amazon.com. (Amazon.co.uk does not show a Help page related to Lending feature.) Where are you located (country) and on which Amazon site is the book?
I believe the amazon.com site is the only Kindle Store that supports lending Kindle books. Also, not all book are lending enabled - you'll see the this option in the image attached. I had to look at a lot of my books to find one that was lending enabled. Most of the big publishers do not allow their books to be loaned.