Yay my nana just booked it for me so im finally getting it

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My nana wasnt sure though on something the lady told her (i wasnt there to talk to the lady) so just thought i would come on here and see if anyone can confirm what it was.
I know i have to bring a bank statement to show where i live but the lady also said to bring a letter. But what kind of letter does she mean? Has anyone who has gotten their full recently had to bring in a letter and if so what kind?