Ok, been there, done that. It took nearly an hour which I suppose only
seemed an interminably long time. They gave me a 'proper' temporary NI number, 'cause they said what my employers (current & two former temp jobs) made up was wrong.

All the paperwork gets sent to Glasgow they said, and sometime within the next THREE months, I'll get my permanent NI number.
I found the process to be a bit of a palaver, really, considering what I was there for. I mean consider: I have already been legally admitted for residence in the country, I am working & have been for awhile, and essentially I'm trying to get this number so I can pay tax, etc as one should. Soooooo - why was this particular interrogation so much more intensive than what I went through to get either of my visas so far - my fiancee visa and then my LLR? I dunno?

We went through all the times that I visited my hubby the year before we were married, and then all the times that I have been back to the USA since we got married -- counting each incidence & the days I was away. I was married twice previously, so she took down each of my former husband's names, when we were married, when we were divorced, and even asked me for their dates of birth! Lordy! I was surprised that I remembered all that! I had the paperwork on the marriages/divorces, but remembering their birth dates was really a stretch! And TBH, I truly don't see how all that is even relevant to obtaining an NI number.

Document-wise she took copies of my passport, marriage certificate, mortgage paperwork, NHS card, employment contract, pay slips & P-45s, Home Office letter. I couldn't remember the postal code for when we lived in the flat across the street

and I didn't think to take mail we received at that address. Luckily, my NHS card had that old address on it. I had compiled several pieces of mail received at our new address, much of what she didn't want to see (other than the items she copied as above).
I guess it was good practice for the ILR visa coming up next year... But just to get an NI number - sheesh!
PS - And Leeds city centre is always crowded to capacity on Saturdays...today with the Christmas shopping season upon us - it was utterly vile.