Student loans - is this letter legitimate?

I've received a letter in the post from student loans company telling me to confirm my national insurance number with my signature and to photocopy the documents that contain my national insurance details, and then send this back to them. I can't tell if this is legit or a scam because I recently updated the university on my student loans application online and I got another letter a few days later saying my application has been approved. But why would they approve it if they 'haven't confirmed my national insurance number'? Has anyone received a similar letter in the post? I tried to call student finance from the government website but they aren't answering.

Hi there,
Is this the first time you're taking a loan from us? What is the address to return the letter to?
Thanks,
Claire

Yes, this is my first time taking a student loan - I'm starting as a first year in September.
The return address on the letter is:
Student Loans Company Limited
P.O Box 4034
GLASGLOW
G53 4XP

Yes, this is our address for the department that deals with national insurance numbers. We check national insurance numbers for all students who are taking a loan for the first time.
Thanks,
Claire

Thanks for letting me know, is it okay if I'm only sending the letter back now? I probably received it about a month ago but I was unsure whether it was real. If I send the letter back tomorrow, will I still receive my student loan on time or will it get delayed?