There is no url to sign up, call the Premium Mail team 0808 100 6778 as it states in the link posted!
It is. All in the link previously posted, if you select Premium Mail from the drop down.
I've finally been able to speak to BT.
The option to sign up for Premium isn't displayed for me at present.
I'm advised that option will only show after 30 days or so - hence my confusion.
I'm advised that BT will contact me after 30 days and, only then, the option to sign up for Premium will be available.