Hey Dowbs,
I ended up just trying to register and there was no problem, I now have my appointment on October 20th! I did some deeper digging and I think the letter telling you to book an appointment (which I never received) is a hangover from the system before it was changed in May.
When I booked, the receipt you get after booking the appointment also tells you to bring the "letter you received from the Consular Section" as well as your receipt saying you have the appointment. I never received this letter and other people who've had their interview told me it doesn't matter, you just need to take your receipt saying you have booked the appointment.
Hope this is helpful!
Cheers,
Joe
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